I don't understand the argument that "we mess up our flexibility next year". That's simply not true. Next year having Amare as a 23 million dollar expiring contract is better than having Gerald Wallace signed on for 2 seasons at 10 mil a year. Nobody wants to trade for that guy now... and nobody will want to trade for him next year.
Next season, trading Wallace will be easier than trading Amare.
Wallace could be traded to a capped-out team that sees itself having a short window after which it will blow things up, so it doesn't mind overpaying Wallace to be a sixth man for a couple of years. Committing to him for three years is tough. Committing to him for a year and a half may be tolerable, especially if the other team has an even worse player making at least $8-9m to send back.
The size of his deal makes Amare almost untradeable, even as an expiring contract, because it is hard to send out matching salary. Expiring contracts are not gold. Expiring contracts the size of Stoudemire's are unlikely to be moved unless the other team is giving up a guy making near the max. Try coming up with a suitable trade involving Amare if the guy you want is making $9-10m.
If a team wanted to trade a max guy to the Celtics for picks during the off-season, I think they would greatly prefer a trade ballast package of the expiring contract of Bass, the unguaranteed contract of Bogans, and Lee (or a Bogans-like sign-and-trade involving Humphries or Brooks or Crawford with only the first year guaranteed) to Amare's contract.
If you want to maximize cap space to sign someone in 2015, then the trade makes sense, but it does mean you have less parts to make the salaries for a trade work out before then. Part of the reason that Wallace and stuff for Amare screws up flexibility is because of the other pieces you have to send out to keep the Celtics out of the luxury tax.