There were no sealed bearing cartridge type bottom brackets until recently so, rather than having grease get all gunked up in the bottom of the frame, cutouts solved the problem. You don't see them on track bikes because grease on the track would be a bad thing.

As for maintenance, I guess that a little frame saver sprayed around the edges couldn't hurt. To be honest, I never thought about it. I don't think much water gets splashed up in there but I could be wrong...