Does the rain get onto it?
Skipping a ride because it's cold enough to cause breathing problems or whatever I can understand, and the problems of ice are famous unless you have a studded-tyred bike, but I don't really understand the widespread fear of rain. It rains sometimes. It's been that way for as long as I can remember. If it's hard enough to hurt or heavy enough to limit visibility or it floods things, fair enough, but what's the problem with plain rain? Stick the waterproofs on and go for it!