Yeah, this has been asked multiple times and originally, before C# launched, Steve agreed that it would be useful. It just hasn't happened so I assume there are some complications under the hood.
It may also be that the problem hasn't caught his interest on the programming side, since Steve usually works on what interests him. I can't speak for Steve, of course, but I usually don't create the same ground units multiple times on a given DB version, and transferring ground units between DB versions isn't doable, so I wouldn't naturally demand such a feature as a player.