Originally Posted by Fat DDRMaster
Ditto's useful isn't limited to revenge killing. It can come in on something like Blissey, use Wish, and then switch out to a weakened Pokemon to recover 150 HP and heal any status currently on Ditto thanks to Natural Cure.
Ditto can also come in on something like Amoongus, Spore one of the opponent's Pokemon, and then switch out to regain one third of its health.
Imposter copies every stat bar HP, so unless you like passing base 48 HP Wishes, the first tactic is useless.