Lou Cypher said:infmach, you don't get it do you?
Let's assume you're playing standard Toxitank WITHOUT Dialga G. I bring up Mewtwo LV.X. You can now do NOTHING to me, except maybe snipe for a pitiful 40 damage with Honchkrow, who I can one-shot at any moment I want. If Mewtwo is my only pokemon in play, then its good game as all you can do is poison me, and I can recover the damage gained from that.
THAT is why people play Mewtwo.