hmmmm, I mean you're putting him in your deck for flexibility right? he might be a 4-drop when you need to play something on 4, he might be a 6-drop when you need a big minion.
You still have to pay all associated costs with the minion he becomes, you have to worry about not having synergies, you have to recognize that he will on average, be average for the mana cost. How many times are you going to wind up with the correct minion to 'answer the board'? or make your turn work well?
I think for a lot of decks there are a lot of minions they would rather have reliably than the potential thing they could get out of this card.
If it was a spell tho.....
Edit: you have no control over what he becomes on a given turn so you're basically rolling the dice over the set of available minions and hoping you get something that is both a good card and applicable in the situation. Seems unlikely and a lot of the time you wind up with a useless card in your hand. You put 30 cards in your deck and chose not to put a different 30 cards because you believed that the cards you chose were the best for your gameplan, why would you give up one of those slots for a card that simply gives you a random card with no control over it's selection?
It would be cool if it was a controllable mechanic, like it was inspire in your hand. Then it could be a great card but as it is I just don't see it.