IGHs are very nice, though you don't see that many on titanium road bikes like the OP is riding. But in my considerable experience of being rained on, derailleurs don't suffer much. The things that suffer most are rims, because the wet debris gets ground into a fine paste by your brakes.

For derailleurs etc the real bugbear is salt, in the winter. That's when I'd take volosong's advice and minutely clean the drivetrain, it's a component-killer.