ARC to THF seems kinda strange, but then, many MMOs in the past have had the Scout-type class that will branch off into either a melee assassin type, or a long-ranged sniper type. The idea, I imagine, is that Archers are the agile, stealthy type, so switching from a bow to a dagger doesn't really seem like a huge stretch, in my head.

However, they should not have you choose between Dark Knight or Paladin; this is supposed to be a game where you're allowed to be every class. Have the DRK quest start after doing some PLD quests, sure, but don't replace it outright. That would be a horrible idea.