That sounds really neat actually. I don't think it'd be very hard to add, either, it's just some hidden assets that show themselves a bit after it starts raining. Roughly the same as river and waterfall effects, just at funny scales. It'd be a rather large amount of work to place all of them if it's a big game with a lot of roofs and gutters, but still a good use of time compared to some of the crap AAA games throw artists at.

Sounds terrifying to try to do in a procedural game though.