There are a lot of good racks available that don't wobble so. The taller that any rack is, the more it will flex, but there are still some quite solid racks with up to five shelves. Look at the better speacialty audio shops.
There are a lot of advantages to using a rack other than the obvious space-saving. You keep the cables shorter, you can ground to a single point easily and distribute power easily, the units each have their own space, etc. You can get racks that have some form of isolation or that are very ridgid or a combination of both. Adjustable shelves are nice for those that are always trying new stuff, but not totally necessary.
-Bill