I think Ainge has done a decent job with what he's had.
-2007:
-Forced to give up the farm to get Allen and KG. Thankfully he was able to do so without giving up Rondo and Perk at least. But to do this he was forced to give up Gomes, who would have been the type of backup 3/4 we want now.
-Able to get House and Posey for cheap. Posey was the ideal backup at the time, the aforementioned defensive 3/4. But let's be clear: it was quite well known at the time that, not liking any offers he was getting, he took a well below market value contract for one year so that he could earn the b.s. "winner" label and ride it to his last big payday. Ainge was never going to give Posey that much money; even Posey knew he was going to get overpaid after a successful year. Posey and the Celts used each other and were perfectly happy with the arrangement; both got what they wanted.
2008
-Tried (and failed) twice to get a pierce backup: Giddens and Walker. Of course, even the best drafters are at crapshoot success levels at that stage of the draft.
-As above, there was no way Ainge was going to sign Posey if he got a big offer. Good thing, too, he's been putrid in New Orleans.
-However, this was his poorest offseason. He should have had a contingency in place/gone ahead under the assumption that Posey would be gone.
-Gave Darius Miles a look at the 3/4 role. Didn't work out
-Had to use part of MLE to keep House.
2009
-Going into the season with Giddens and Walker older and no first round draft pick, they opted to use the full MLE on Sheed as Perk and KG both have injury histories and Davis is quite undersized as a C. Hard to argue too much with that.
2010
-Not sure there was much true 3/4 talent out there that was available. I would have liked Rasual Butler, but he opted to stay in LA. Full MLE had to be used for a PF/C with Perk's injury; maybe they would have done differently had they know Shaq would fall into their laps?
Essentially, it's really easy to look back now and say "man, i wish we had locked up a solid 3/4 backup for the past couple years," but really hard to address when you have to rebuild the team for each season at a time, without knowing how things will shake out.
Remember:
-C's have one of the most disproportionately "starter heavy" pay rolls in the league. None of us would be shocked if LA moved Bynum or Odom to make the team better; Ainge would get absolutely skewered if he dared trade one of our starters to deepen the team. Heck, we get skewered for suggesting Davis trades. So when you clean house to make a great starting lineup and are not allowed to trade any of those parts, and to get those parts you a) built a good team so you got bad draft picks, and b)had to trade some of those picks for the starters anyway, you are left with rebuilding the bench through MLE and vet min contracts and bad draft picks. I think we as a fan base fail to acknowledge how difficult that is, as well as drastically overestimating the chances the C's can sign whatever free agent they want for the MLE/Vet min when in reality, the player sees 30 choices of where to sign for that type of money and really doesn't have the Pro-Boston bias that we all do, and instead is thinking about playing time, friends, family, schools, weather, taxes, etc.