When I train in bad weather, it makes me wonder if the other guy is sitting at home. It makes me more committed to the ride. It's more of a challenge. You get to be part of nature, rather than watching it from a window. Does it make a difference on race day? Maybe, maybe not. I like to think I get a few extra training days by not skipping a ride. If its raining on race day, I have the experience of having been on wet roads maybe more than the other guy.