Without needlessly repeating everything that's already been said, I think Leon's injury problems preclude this from being a valid argument ... we just did not have the financial luxury of having another big on the bench that wasn't seeing court-time. I loved Leon, and in fact he was a much bigger part of Banner 17 than most people give him credit for, but not signing him was the right decision, and the production we're getting now as a result backs that up.
We don't have any big men on the bench who don't see court time. We have numerous non-big men on the bench who don't see court time. Knowing Leon's unique capabilities as a post scorer (something we lack, particularly if sheed can't play in some games come spring -- he's no spring chicken, and has the biggest gut on the team) I feel it's worth asking whether he might be useful when the games count most -- in the playoffs -- when guys are worn thin from the grind of the season.
If we had useful players throughout our bench (say, quintin ross instead of jr giddens or ime udoka instead of bill walker), i might be less inclined to make this argument. but we don't. Lester never needed to be signed over leon, and had we needed to cut jr or billy to make room for a vet in order to insure best chances for a championship, it would have happened at or after the deadline anyway when the majority of their salary would've been paid, making the financial ramifications minute.
One of our biggest rivals in the EC felt it was a worthwhile risk to add leon for small money across a year and a half. Considering the problems we had without leon last playoffs, I happen to agree with them.
As fafnir points out, leon walked out of his meeting with ainge with hurt feelings. if the Cs couldn't have signed him for the same deal the Cavs did, so be it. but clearly it could've been handled better.