You know, I think with perfect hindsight it's all so easy to say signing Wallace was a bad move. But let's try to keep some sense of perspective.
At the time of the Wallace signing, the Bulls were in dire need of a solid big man. Tyson Chandler was NOT the player he is today for the Hornets. Joakim Noah and Aaron Gray were NOT on the roster. Tyrus Thomas had just been drafted and was considered a project. The Bulls were getting killed in the post. Fans were clamoring for the Bulls to acquire an established big man. Ben Wallace was clearly the best free agent big man that season (if not the best free agent outright). The Bulls had cap space they were going to lose if they didn't use it that summer. Someone else mentioned that Wallace was considered the heart and soul of the Pistons, a division rival. Signing him was a chance not only to improve your team but also severely hurt a division rival. Everyone knew at the time the Bulls were overpaying, but given all of the above, it was still generally considered a good move.
The only downside, really, is the one that it turns out has bitten the Bulls in the backside-- the aging of Wallace and subsequent decline in his play.
Whether or not there were valid, logical reasons for making the move is not relative to whether it was a good move or a bad move.
It has turned out to be one of the worst free agent signings in recent memory. It has been an absolutely, atrociously, horrible, awful move. Especially given how clear it is that the Bulls are going to be a penny pinching franchise. If they were going to spend money in a fair ratio to the amount of money they make vs the rest of the league then it wouldn't be magnified so much.
This isn't to say that the move wasn't made with some logical reasons. I'm not saying that the decision to make the move was nearly as bad as the move turned out to be. Expecting Wallace to decline from defensive MVP type player (even if he won it largely on notoriety rather than ability) to guy who is lazy, doesn't give a crap, and is a below average big man couldn't have been anticipated. I think even his largest detractors figured he'd not reach that level until year 3 or 4 and that he would have a more gradual decline.
I don't feel as bad about the decision to bring in Wallace as I do about how poorly it has turned out if that makes sense. I can see the logical reasons for it, especially given how Chandler absolutely whithered his last season here. If we were going to play moneyball (which it seems that we are) then it was a bad move regardless though. You don't overpay a 33 year old guy if your franchise is ultra cheap. You can't make signings like you're the Yankees if your payroll is really in the Tampa Devil Ray range. It seems apparent in retrospect that the Bulls did this. They knew they were cheap, and they knew they needed to save money for Gordon/Deng, but they didn't do it.
I never believed it would happen before, but Ben Wallace will likely cost us Ben Gordon in the off season.