this+purify=great defence.
And purify is cheap and easy to splash in decks, so it would be amazing. Im not sure that a mechanic like this is balanced(i mean, make all damage poison, then pack purify and it does not matter what happens, you will always have a chance to heal back to full(in the turn they give you the poison, you take no damage, using a purify next turn prevents all the damage by destroying the poison counters, or thats what i understood).