Things that limit diversity substantially need to be banned
Sure, let's go with that. Goodbye, Stealth Rock! Goodbye, <insert any Pokemon who outclasses several others>!
Things that offer a significant advantage to the user at no cost need to be banned
This statement doesn't really mean much of anything. We may as well ban the 4th moveslot under this rule. Or egg moves. Or items. Or any ability that doesn't suck.
As I've said before, we don't ban things because we are punishing them for being "too good". That would be stupid. We ban things because they make the game unplayable.
Anything that reduces the metagame to "team matchup" needs to be banned
Perhaps I'm just not understanding you properly, but it's been impossible to create a team that counters everything for years and years now. Which means that matchups are a major part of the game. If you really want a game where matchups and luck don't matter, go pick up chess.
1. Drizzle makes Fire types unviable This one is obvious. With a 50% decrease in the power of their STAB moves, Drizzle ruins the viability of all fire types. It is definite that without Rain being so common, pokemon like Heatran could make the top 5 while Infernape and Volcarona could become the top-tier sweepers they wish they were. It is also wholly possible that pokemon like Arcanine, who is very powerful in UU and has many new options in Gen V could make low OU. This is especially obvious with Darmanitan, who would normally be much higher but is now placed low at the #53 spot because its only good point is its ultra-powerful Flare Blitz.
Fire Pokemon have deeper problems than Drizzle. As you said, Darmanitan's only real selling point is that it has a stupidly powerful Flare Blitz - anything that one-dimensional will have a hard time excelling in the OU metagame, Rain or no. Fire's a pretty mediocre defensive type to begin with (weak to Ground, Rock, and Water isn't a great place to be), so it's not surprising that there aren't a ton of them floating around, especially when the offensive coverage isn't ideal, either (aside from Steels, the other things it hit Super Effectively aren't that common).
Mostly, I suggest you just plain don't bother trying to figure out what the metagame would look like without Rain, because you
will fail. Metagames are fickle, unruly beasts, and it's impossible to predict the exact results of any significant change. It's sort of like predicting the weather or the economy in that regard.
Not to mention that this exact argument could be used to ban Stealth Rock, since it creates a metagame that severely favors SR resistant Pokemon and severely punishes SR weak ones.
2. Drizzle makes sandstorm more viable then they already would be. We all know how great sandstorm is, and we all know that it is the dominant playstyle in OU. One thing that many people don't realize is that it is only so great because it can counter rain. For players who choose not to use Drizzle, there is only one other option to remain viable in OU. For reasons I will explain below, non-weather teams are pathetic when compared to any type of weather. The fact that Drizzle encourages the already most common pokemon in the game is simply unforgivable. Soon we will reach a stage where Tyranitar will be similar to Scizor in late Platinum OU, where I think he had over 30% usage. The game will revolve around Tyranitar, Excadrill, and Gliscor. Not good for diversity, or for balance.
...Huh?
This argument is quite the reach at best. By severely weakening things that clearly have an advantage over them, Tyranitar, Excadrill, and Gliscor will see less play? I'm just not seeing it.
If any type of weather beats non-weather, than there would, by extension, be even more Sandstorm teams to fill the gap if Rain was gone. Especially since Sandstorm does not counter Rain very well at all; on the natural Rock/Paper/Scisors of weather wars, Rain beats Sandstorm (due to Rain abusers/inducers having natural advantages over their Sand counterparts). While this matchup priority certainly isn't an absolute (particularly at higher levels of play), saying that Drizzle encourages Sandstorm specifically is a questionable argument.
Indeed, what's stopping me from saying that it isn't the opposite that's true? Perhaps Rain sees more play because Sandstorm is such a common force in the metagame. Something being used to counter something else is much more plausible when the something else is more common than the something, no? If Sun counters Rain counters Sand counters Sun, the logical assumption is that the abstract best weather will see the most play, followed by the counter, followed by its counter. So Sand > Rain > Sun in that scenario. Which is what the data would indicate.
3. Any Pokemon that can't abuse rain (or other weathers which counter rain) is unviable. I want you to pull up the May 2011 usage. Read off the top 20 Pokemon in OU. Only 2 Pokemon, Conkeldurr and Gengar, lack the ability to abuse weather. More than half of OU gains a boost in usage, while the other half gets a decrease in usage. Its like taking money from the poor and giving it to the rich; and its all caused by Drizzle for the most part.
This argument was crap when Nkululeko made it, and guess what? It's still crap. With a couple exceptions, the top 20 in OU are the top 20 in OU because they are good. Not because they are weather "abusers". If you can seriously claim that stuff like Ferrothorn and Reunculus wouldn't see just as much (if not more) usage in a weatherless metagame, then I don't think I can help you.
And just to piss Nkululeko off, I'll say it again: Correlation does not imply causation. Just because the top 20 are mostly composed of Pokemon who can do well in weather does not mean that you can say that the top 20 are used because of weather. Nor can you say that weather is used because the top 20 are good in it. Those are unsupportable conclusions.
And "it's all caused by Drizzle" is an incredibly silly assertion. You really, really need to be careful not to make arguments that start at the conclusion and go backwards, especially when past evidence clearly refutes it. As you yourself said, Sand was the dominant force last generation, with many of the most common Pokemon being extremely good in it. It is incredibly careless to say that it is common this generation because of Drizzle.
4. A Drizzle team has an automatic advantage when facing a team without weather. A Drizzle team gets many advantages that a normal team doesn't have, and can build its movesets accordingly. For one, it has double STAB on water moves. This allows is to hit everything, and hit it hard unless that thing's name is Ferrothorn. Secondly, it can avoid Fire moves on its team. In this scenario, the Drizzle teams saves several moveslots that are wasted on the team of the opponent. That can mean all the difference. Rain also allows for 100% accurate Thunders and Hurricanes. Let's say that two Starmies, at full health, are facing off for the win. The one on the rain team will succeed simply because it has that free 20% extra damage. Think about it this way: if two equally good players battle, the one will rain will more likely win due to these advantage raising his/her odds.
5. Drizzle has no drawbacks, only positives. There is no reason to NOT use Drizzle. It can beat sandstorm, gives many advantages, and can neutralize potential threats in Volcarona, Darmanitan, etc. There is no drawback. Unlike sandstorm, it can't hurt your own Pokemon.
Since these are really the same point, I'll just argue both by saying that it cuts both ways. By running Drizzle, you are making it harder on yourself to counter Steel, Grass, Bug, and Ice Pokemon as you cannot successfully run Fire attacks to counter them. You don't "save moveslots" this way, you lose options. Also, your Water-neutral Pokemon are suddenly taking large amounts of damage from incoming Water attacks as well, so your Scizor can now be OHKO'd by strong Water or Fire attacks instead of only Fire attacks.
Then there is the fact that you're spending one team slot on the mediocre Politoed, the fact that this mediocre Pokemon needs to be sent in early, and the fact that you need to tailor your team around Rain in order to make it work (albeit to a much lesser extent if you are simply using Politoed as an anti-Sand/Sun Pokemon, as many people do). Basically, by running Drizzle, you've just made your team that much more predictable.
While I won't insult you by claiming that Drizzle doesn't have serious benefits and few drawbacks, it's not like any given Rain threat cannot be countered, especially with the worst offenders (the Swift Swimmers) out of the game.
6. A Drizzle team, when facing a team with one or two fire types, is more likely going to win simply due to team matchup. Lets say that I have a team with two fire types. Neither have a weakness to Stealth Rock, and both are perfectly viable in OU like Heatran and Infernape. In the rain, I essentially have two potential deadweights. Its like the score is 6-5 right off the bat. Or lets say that I am running a Drought team. I am already at a disadvantage due to the fact that Grass types don't even get a STAB boost in the sun. Like above, if I am just as good as my opponent, I am more likely to lose. This reduces the entire metagame to luck, and those who run into more teams that they are effective against can do better. It was not like this before in Gen 4, where anything could be played around. You cannot play around Rain. It will always be there, nerfing your Pokemon and boosting theirs.
This is basically just point 1 all over again, but I'll echo the question of why you're using two Fire-types on the same team without being Sun. As previously stated, Fire is a mediocre typing anyways.
Ultimately, I'm starting to agree with some of the whiny anti-Smogon people on sites like GameFAQs. Not for most of their unsupported hate, but I do agree that we are perhaps becoming a little too ban-happy. Our goal should not be to craft a perfect metagame, since it's unfeasible and we might as well start from scratch and make our own game. We should really try to ban only the things that pose a serious risk to the game being fun and competitive, not simply because things are "broken". If you hadn't noticed, Generation 5 made everything and their mother broken, so if we ban everything broken we won't have much of anything left.
Once we've obtained a workable metagame, we should stop banning stuff. If there is nothing around that cannot be reasonably dealt with, the metagame is at a point where future bans cannot guarantee a better metagame. And there's nothing in this metagame that cannot be reasonably dealt with, including each of the weathers (if the solution is to save a space on your team for weather of your own, then so be it - we should not ban something because players are unwilling to counter it). While the metagame isn't perfect, it's plenty fun to play so long as you aren't going to whine about the weather.
I'm starting to think that a separate Clear Skies tier might be the best solution if it means we can stop having these petty squabbles. If it means that we can finally let the metagame settle instead of banning something new every couple months, even better.
And for what it's worth, weather is inherently centralizing. Not perma-weather, just weather in general. Weather abilities are extremely powerful, and the weathers themselves give a significant boost as well. It should come as no surprise that basically every metagame that has at least one decent weather inducer uses it, while pretty much every other one sees significant abuse of Damp Rock abuse. Only when weather is nerfed into oblivion can weather be ignored.
