I wouldn't have pulled the trigger on this deal if I was Ainge either, unless we were blowing it up (trading KG as well) and I knew that Smith would sign an extension at less than the max in order to play with his best buddy Rondo going forward.
I don't think swapping Pierce for Smith would bring us any/that much closer to beating the Heat or Thunder/Spurs. So to move the Captain to a sub .500 ball club and still not make us legit contenders doesn't seem right. Plus you just know someone out there will be desperate enough to offer Smith a max contract next year, which I wouldn't be comfortable giving him, so he would need to be willing to take less money to stay (a definite possibility, but you risk having him walk at the end of the season if you don't get a guarantee from him).