the problems with playing as Exile or Revan is as follows:
First there is the fact that they'd have to come up with some reason why your character who could make the universe tremble at his approach is suddenly a green behind the ears novice.
Second, Revan and Exile are both established characters. It may seem alright to have players re-play these characters, it's unfair to new players to the series to be forced to play them. In Kotor 1, you were Revan, but you had some ownership with the character because of the amnesia. In Kotor 2 you were the Exile and given free reign to go any direction with molding that character. The problem lies in the fact that once each of those games were done, the main character was a certain way.
Third, The exile and Revan both left the known galaxy. So we may be able to follow their story lines, but at the expense of the fate of the Jedi order and the republic. Those storylines, which i feel is the actual main arching story of the series, would be left behind.
Fourth, If you play Revan the Exile fans will be pissed, if you play as the Exile, he Revan fans will be pissed. If you play them both, then you lessen the experience as you spread game "self" too thin. With a third PC you piss off both fan bases, but you're able to equally tell each storyline well while maintaining the players sense of game "self" (the emotional investment in the creation and forwarding of a character persona). You also allow the game narrative to look at aspects of past conflicts with a fresh perspective and see them in a new whole.
Fifth, Each character comes with a certain level of character attachements. Revan has Bastila, Carth, Jolee, etc etc etc. The Exile has Handmaiden, Atton, Visas, Mandalore, etc etc etc. If you don't use the NPCs the main characters are attached to then you lessen those characters. If you do use them then you greatly limit the creative freedoms to explore new NPCs or fresh storylines.
just my thoughts...