It's not clear that tread helps with traction on bicycle tires in the rain at all.

Here's a detailed explanation.

(And a bit more to explain why bicycle tires generally don't hydroplane, and Sheldon's thoughts on bicycle tire tread.)

As I see it, on a bicycle, slicks are ideal for anything that has you on pavement or of hard packed dirt. Only if you spend significant amounts of time on sand or soft dirt or grass or something like that would I think that any sort of tread would be a good thing.