I don't know if the engine has the capacity to alter trajectories of ballistics for something like that, but seems like it would be fun in moderate effect.
Accounting for windage could make for some interesting mechanics, esp. on Sandblasted, with the wind having more effect on the trajectory over range.
But would that make everything just too hard to hit with ballistics and make them useless at long range?
I would vote against wind effecting weapons if that was the case.
I was playing L4D2 last night, and the weather effects in that game are placed there to change your visibility and situational awareness.
I think that using this in MWLL would be fun, possibly tying it into the day/night cycles that maps already use.
Like a coastal map that is foggy and rainy for a while in the "morning", clears up for a bit, rains in the "afternoon", etc..
Some maps have touched on this aspect in having a persistent weather effect, but is there some way to make it dynamic?