This isn't a question of how good Yao has been. If Yao can come back healthy and play, then he's worth what he's being paid. He's not a superstar as some would like to make him, but when healthy he's one of the top five centers in the NBA. Having said that I will acknowledge that there aren't that many good centers in the NBA right now.
The question is, will he be able to return to his former self. I don't have the answer to that question, and without that answer, I wouldn't be willing to take a risk. He's owed around 16 mil next year, and thats a big chunk of change to take a flyer on. So for the moment I'll pass. He could be everything we need, but he could also be the next Ralph Sampson.