I find this frustrating as well, mainly because it costs no more to develop 220 over 120 at my lab. When I have 220 in a camera, I feel less concerned about taking a shot that isnt perfect. 30 shots in 645 is very freeing. Mistakes dont bother me as much, and I will take extras just to make sure I got a decent shot.
12 shots is OK, but 24...again, limits that preciousness of each frame.