would be nice if they turn on in the rain or if mud hit the windshield.

Well you don't really need wipers in the current state of the game, since the windshield doesn't get muddy or wet from rain, only the occasional splash when entering water, but it clears up quickly. Focus has already said there will not be functional wipers. But they sure would be nice, as long as the vehicles actually get wet from rain (which they don't).