The problem is, you're getting into a can of worms. Remember Squiddy, the giant pizza/beer bottles, and UFOs? I bet those were all custom objects at one point. But, that's five objects. As soon as one guy makes a custom hangar, then a custom runway, then there's someone who will want custom fields, then buildings and trees...and, IF that person has decent 3D skill, your installation size has just doubled. If not, tripled or quadrupled, not to mention the bugs that may come about. And who's gonna get blamed? Not Joe Mapmaker.
And that's just one person. We have a community of mapmakers, each capable of making MULTIPLE "world sets..." You'd have to dedicate a hard drive to HTC.
Even if you try to limit it to, say, random objects like Squiddy, someone's going to whine that "he can but I can't, all I want is _____" and we're either pretty much back to here or starting that Cycle of Doom(tm).
