Commenting on the last statement:
It's because people are playing competitively and only care about winning. I feel that creativity should be used too (commin from gal who trolls rain with specs Blissey), but I suppose creativity isn't used much since most ppl in tours or on ladder only care about winning :x
Also, no matter what we ban wont diversity always be limited? (Thus the definition diverse imo, how is banning rain / sun going to increase diversity when there will just be another common archetype to replace it? )
Banning Drizzle / Sun will only lead to these competitive players on leaning on other things that work anyway. Therefore, I don't see how diversity will be effected. Now whether auto weather is broken or not is a sticky subject, but I don't believe in its diversity argument.