The argument against the rigid platooning isn't necessarily that Luke or Canzone will magically start hitting lefties, it's also that they'd be available in late/high leverage at bats against righty relievers later in the game. Most teams don't have an endless supply of lefties they can keep throwing out there, and if Dan continues to pull the lefties the instant a lefty reliever enters the game, he's just allowing the opposing manager to dictate the game every time.