Well, to use an example from 4th Gen, where rain was very similar, but lacked Politoed - how common was it? It was viable yes but by no means common, which in general can be taken as it not being a dominant playstyle. Now think that this is the Rain that we reduce it to if we ban Drizzle - and consider the other vastly improved weathers which on every turn can reset the weather with a mere switch. This is why I see Rain as being unviable.
In any case, it's not merely the Damp Rock, extra moveslot and turn count that neuters Rain without Drizzle. The fact that a turn must be used to cast Rain both makes the abusable turns 7, and gives the opponent a free switch or setup oppurtunity. It hinders your switching, because doing so wastes Rain turns - so if your sweepers can be walled, clever switching can stall them out until Rain goes down and the opponent counterattacks. With Permarain, things can repeatedly be worn down with little fear of Rain dropping. These are some of the non-tangible reasons why Rain without Drizzle is much less effective.
I concur with the other guy who responded to this - I don't necessarily agree that banning Drizzle, or even its abusers is the way to go at all. I agree that testing Manaphy primarily then seeing how Rain is afterward is the way to go. But given the amount of people advocating Drizzle's outright ban and brokenness, I felt that I should defend it - and in the event that something is broken about Rain without Manaphy, I was suggesting that it was the abusers and abilities which were more at fault than the weather itself. Admittedly I gave few reasons initially, but this would be because I do not necessarily believe it to be true.
