Think a lot depends on the frequency of being wet and the length of time between rides. If you're storing it, put some sort of water proof cover (don't forget about pest if there's any possiblity). If it get's a little wet now and then, doesn't seem like it would be any different than washing it unless it's could sit for days being wet.
Anything that's steel will rust. Most of the fasteners will rust. Brake rotors. Might cause connectors to corrode prematurely. Water getting and sitting around places is usually undesirable, can expose wiring issues that normally wouldn't surface (this one isn't very likely, but it does happen, and when it does...).
I don't think water is involved in formation of aluminum oxides (steel/aluminum seizing) but it might be?