Over healing should definitely cause threat, there are many reasons, but two that jump out at me are scholar with their shields, many times scholars will preheal for the shield to prep for a big hit, even if the target/targets are at full health, this would turn it into effective health via no threat. Secondly there are Hot's if you stack more than one heal over time effect, they stack, if the total tic heals the target and then some, who gets the threat for the amount healed, and who gets credit for the over heal, do you split it in half, do you have the game do a calculation and break it up into percentages based on who provided which portion of the tic?