I've been commuting for the first time this year on a singlespeed and in the last week have experienced my first two days of commuting in heavy rain. It occurred to me almost immediately that a slightly lower gear for rain would be a good thing. Only because I need to go more slowly in the rain but would like to keep the same fairly vigorous cadence my 42x16 regular gear allows. In my case we don't have a reliable rainy season - winter means snow, and singlespeed riders here advise not to gear down just for that, because snow can increase traction and a lower gear can slow you too severely. But if I were in a climate where it rained much of the winter I'd definitely have a lower gear.