Regigigas would be usable then. Trick would be great with the item also. Nerfing pranksters or pokes that really rely on their ability. Ferrothorn without iron barbs, ninjask with out speed boost, ditto with out imposter, etc.
Hum, I do hope an item like this exists one day then, stopping prankster leads cold with a switcharoo (though taunt already does this), or being able to actually use regigigas in singles would be nice.