The deal is fine. This basically ensures that Pierce will not be traded or amnestied. If it's a 2 year deal, with a player option for a 3rd, it will expire with Pierce's, if he doesn't opt in. If it's a 3 year deal, and he does play all 3, it will be expire with Rondo's.
This also means that we will not be looking at the FA market the next 2 or 3 years. Ainge said that is just a recipe that doesn't work. It's only worked for one team (Miami), as most players sign extensions and don't ever make it to free agency. This means that the likelihood of Allen returning for a 2 or 3 year deal just shot through the roof. Just like I now expect Green to sign a multi-year deal as well.
If Allen does return, I expect us to target a true center with our MLE (Kaman) in order to preserve KG. So a 3 year deal for the just turned 30 year old 7 footer, at yearly increases each year seems likely.