It's pretty simple. Yao has the ability to carry a team, but has not shown that he is able. Tmac has and does.
With T-Mac, he is able to single-handedly take over a game. With Yao, you're going to need to have effective shooters and passers who are able to get the ball to him down low. Unfortunately, as last night proved, that is hard to do when the shots aren't falling.
Rockets are not nearly as good as they should be, people started to argue about whose contribution is bigger to what success? What if we actually win a championship, those who laughed at the Lakers so hard, will they start to laugh at themselves even harder? Why is this question even necessary? None of them can carry a team by himself to success. TMac already proved it, and Yao can't prove. That's why we got TMac at the first place. There is a Chance, if they really gell, they can win eventually. Now what's the point to argue whether the team is No. 20 or No. 25 with or without one guy?
You can say that for all inside-outside tandem. The big man, by the nature of his game, always relies on his teammates. Even Hakeem couldn't win if his teammates couldn't shoot. (Yes, I know it's blasphemy, and no, I am not comparing Yao to Hakeem.) The perimeter guy, however, can "single-handedly take over a game" because he is mobile and controls the ball. That doesn't mean the outside player is more valuable than the inside man. Back to the question about TMac and Yao, I really hate this pitching two teammates against each other comparison. We've done that with Francis-Yao, now it's TMac-Yao. What's the point? Everyone knows TMac is the better player.
It's simple for a few reasons. 1. T-Mac is a guy who can carry a team. He's the guy the Rockets go to in crunch time. 2. We don't have a decent back up small forward, but we have Mutumbo and Swift able to play center if Yao is hurt.