Unless they make a point of you waking up the first and/or every time (and having you influence his target), there's still the question of who he targets (random, does he target certain characters more, does he target wounded or healthy more) and how much he drinks (when you are controlling him there's a chance to botch a roll and accidentally kill after all). That and the question of what you do when you find out (and whether you're the only person who finds out at first, or whether it defaults to everyone finds out). If you find out first and keep the secret, how much grief does that cause when everyone finds out (I doubt it's possible to keep it under wraps indefinitely). For that matter, can he feed on enemies? Does the "feeding while camped" mechanic get bypassed if you have him scout ahead, sneak up on some goblin and drain the bugger?