I've seen a few bikes with drain holes. I don't think it's common practice though, since it's more likely to introduce water and dirt that drain it. I certainly would not go drilling holes in the bottom bracket, either. If water is being introduced, it's likely through the frame tubes, and with regular use should exit in the same manner. It never hurts to inspect the bottom bracket after a wet ride, BTW. I found that out the hard way just recently.