I didn't even know this was a thread till just now (thanks
Sae Sae signature).
I am on the keep m-mawile free front because there seems to be alot of prediction free talk here. Often times on the ladder people predict my moves a few steps in advance (I'm too predictable I guess), so why can't people predict the most predictable poke in the game?? Mawile comes in on setup fodder, so you can do the seemingly forbidden double switch to punish that switch, right (am I crazy)? Or mawile comes in when screens are up to tank a hit, so be prepared and dont let it switch in without cost and not wanting to set up anymore. Also, if SD does get up, you can switch back and forth or use status moves, making them predict a play rough/whatever, it isn't as outlandish as many are making it out to be (that is how I donk most m-maws and I am doing OK).
The Commoner: Well between rocks and spikes arvada, that is impossible as every time you try to defog or rapid spin them, mega maw is going to switch in (fodder scenario I'm seeing) and smash something. Or if you try to keep mega maw from sweeping, something else will (fairy having many options scenario).
To answer the first case...
Why did you let rocks / spikes / both get up in the first place? They aren't automatically up when the game starts, what were you doing with those turns? I don't like arguments set up where rocks and spikes are automatically in the calculations, because you are assuming nothing was done by the other player during that time, such as killing an important poke (thus something is dead and less options are available for the fairy user later), making sure those things didn't get set up with a defogger/ rapid spinner staying in, or a bunch of switches happened. I can't generate every scenario, but I can say FOR A FACT, that the spikes / stealth rocks did not get up for free, and if they did, it was 1 layer of something on a good switch (if more got up during 1 play, there was human error, ie not switching to a counter). In the same vein, not every spinner / defogger is setup bait. Armaldo, excadrill, and a few others can EQ, tentacruel can scald then switch to quag if the burn attempt failed, zapdos can heat wave, and skarmory can whirlwind/ roost with rocky helmet. Not every team is going to have these pokes, but they are definitely common, and if I need to list more I can find more standard pokes that can deal with this.
To answer the second case...
Typically, I see klefki, togekiss, and gardevoir on most teams as pokemon that aren't going to sweep and have some other purpose. Such as scarf tricking, healing wish, spikes, what have you. Then there is diancie and clefable who COULD, not always, be setting up rocks. So about half the team is a bunch of setup and death fodder to get azumarill / mega maw / calm mind or cosmic power clefable in to sweep. These pokes are not only slow, but they WANT to set up
desperately. Every azumarill I see wants to belly drum like a maniac at full health to sweep, so attack it or, if something looks like it wants to die and you can't stop azumarill with the current pokemon out, don't kill it and let azumarill set up for free, because you should know that is the plan. Mega maw is the same way, you need to play fairy like a chess game. Sometimes they can move these "rooks" into position where you are in checkmate, but you could have made a move earlier to prevent this, such as keeping pokes in that they can't set up on. Clefable is a bit different, if you have nothing to stop it, you really have nothing to stop it. Everyone magically has this crazy gen 3 clefable with softboiled so after a few defense boosts you aren't getting through it. To add to my other statements, I'm aware that between azumarill and mega maw sometimes you don't have something that threatens both of them and the poke currently out, but it seems to me that most generic teams have answers.
I'm not naive, fairy is one of the toughest matchups I have to play against. And there does seem to be a trend of people thinking fairy as being an easy ticket, among other types, to hit the top with a great gxe and seemingly no skill. However, isn't it amazing that the same players and same player's alts end up at the top regardless, even without fairy, or mega maw? And outside of arken and ashaury, and I'm sure there are others but IDK of them, I can't think of many notable fairy users.
So in the end, my suggestion is plan ahead in team building versus banning m-mawile. There are some people making crazy sets for unused pokes to stop mawile, but that seems to be people looking for a 100% counter to mawile and fairy cores in general (the poke probably doesn't exist, at least for every type) instead of making plays. I don't think that not wanting to make plays is banworthy. Mawile is the bear in the monotype woods, if you choose to not bring a shotgun with you, THE BEAR WILL GET YOU. Since the beginning of pokemon there have always been pokes more effective than others, and in my opinion, mawile is just a good poke to be prepared for, just as much as charizard-y and other behemoths.
EDIT: I didn't address it on steel mainly because I honestly don't see it being used over mega gross (ever, I've seen it a handful of times on the countless steel teams I have faced), if you guys care I'll address it (I don't think it is a bigger deal on steel than fairy, if it is a deal at all) but otherwise these are my thoughts on the matter.