I don't see many advantages. If you're only letting them stack three levels deep, you could've done the same thing with a wider road. Some other issues are that people can't see underneath or behind,or really overhead, so when someone blows past them from below or above they'll probably find it disconcerting. Our visual system evolved primarily to track threat motion in the horizontal plane. We'd probably get used to it, though. We can get used to almost anything.