Worse than rust for me was the increadible amount of GRIT that you pick up from the road when you ride in the rain. That stuff gets everywhere into the drivetrain and is a bitch to get off. As often as you wash the bike it's not going to remove all of the grit. Some of it will migrate into the chain and other moving parts. Once it meets oil or grease it becomes like gooey sandpaper.
I don't think a bike never having seen rain is that big a deal, but it's certainly better for a drivetrain to never be exposed to dirt, grime and grit like you get when commuting every day.
I no longer
have to ride in the rain and I avoid it whenever possible.
