It focuses it's use down. With the Over Zealous card, you have to kill it with the paladin. Which means you either have to widdle it down a lot, or have a lot of them on the field. But that makes growth creatures very killable, and it helps defeat things like malignant cells in one fell swoop.
That doesn't make it less OP, but it
does make the card focus more on what you originally wanted it to do. Another upside, purely using the card, killing the creature will cost
mana equal to it's toughness, like you wanted the card to.