After you break all of the grass for seeds, the only way to get that grass again is if you use bonemeal. What if the rain gave the grass blocks a chance to spawn grass, flowers and mushrooms to spawn? This would only be like a 1/100 chance so there wouldn't be grass and flowers everywhere. It might also give you the opportunity to find either a red or brown mushroom you can't find anywhere else. Having a bare forest undergrowth after breaking all the grass doesn't look very natural...
Regrowing Grass, flowers, etc.