Yeah but XII's characters were TOO changeable, thanks to the rather worthless Quickening system that rarely did the damage it was supposed to and not really being all that difference power wise between characters. V got away with it because you only had those 4 characters the whole game (technically
). VII got away with it because Limit Breaks were so diverse and allowed you to cater playing styles to their uses. VIII also had very useful character specific limit breaks that were all useful in many different circumstances. X did not allow that level of customization until late game (well, early if you are in the International version).
I suppose it comes down to if you like to create a static party of 3 or not for your adventures.