I'm deeply saddened that 4 days away from launch people who have obviously preordered the game don't understand that your advanced class is functionally different with different abilities and different core mechanics then your base classes other AC.
For example a Bounty hunter has 2 AC's; Power tech and merc.
Powertech; doesn't use cover, has a lot of shortrange/melee abilities, only uses one pistol and can dps or tank
Merc; Uses cover, has mostly long range abilities, duel wields and can heal or dps.
For the feared wow comparison this is like saying you want to change your character from a hunter to a paladin because 'they have the same story lol'.
They are entirely different classes with different talent trees, abilities, playstyles and mechanics you can't change your class halfway through any other mmo why should you be able too in tor?
Merc's don't use cover.
Now, the comparison you raised is accurate, but it doesn't actually make an argument. Okay, the OP is asking for the option to let his 'paladin' be changed into a 'hunter.' So, why not? The only reason you've given is that no other MMO does that, and I think even you will realize that's a pretty weak reason.
So, now to think of a reason for it. Here goes, it would increase player enjoyment, thus increasing player loyalty and the overall health of the MMO.
Okay, balls back in your court, now come up with a GOOD reason to disallow AC switching.