I didn't mind the playing as other character parts, but I still would have rather been able to pick my party, ala BG (sorry, it's the game I'm currenty playing so I'm referencing it a lot). It really does add to replayability.
Although I suppose games like KOTOR are much more reliant on story driven moments than BG, so there's a larger chance that the story would be broken or key moments passed over if you didn't have the exact members of your party around at a given time.