I hope Mutu stays, primarily because we got him for free while Cassano would cost a lot, something we wouldn't afford unless we sell one of our star players. It's true he hasn't played regularly for a long while and that it would be impossible to know whatever to expect from him, but what if he regains his form again? That would be much easier for Juve, as the club wouldn't have to change much in the squad and would at the same time have one more quality striker to chose from.
On the other hand, there is the doubt to weather he will regain his form. After all, during his stunning season with Parma he played regularly and was given a leading role, along with Adriano being the most important player in the team. At Juve he has 4 quality strikers to contend with, which means he will hardly see much action unless injuries play their part.
The question is therefore: will Mutu be able to regain his form or not given the current situation? I guess that is quite impossible to know but nonetheless I'm hoping he will stay.