But if it's divided, then why would anybody think guarding the healer was useful? I already thought it was useless before I realized the point about dividing.
Yes... exactly this. Makes no sense except in very unique situations. None of those situations can be found in and heroic, flashpoint, Eternity Vault, Karagga's Palace, or Explosive Conflict.
At the moment guarding the healers only makes sense on Terror from Beyond 2nd stage and PvP.