This team is obviously a contender, and with most players (sans West, Perkins, Davis) on board for next season, they should be elite next season too.
Why risk ruining a title chance for a slim chance at a good player. D12 is pretty great defensively (I still prefer KG's all-around defensive presence for this team more), but he offers little offensively still. He worked with Ewing his whole career basically, so a couple weeks with Hakeem can't change him all that much.
Ask the Knicks what happens when you sacrifice seasons for a potential FA.
Also, at $27M for Rondo and Pierce ($29M with AB), even if you are able to convince Howard (at ~$13M+ starting salary) to come here, that is not a lot of money to fill out the team, unless you have KG/Ray come back on vet minimum deals. We would lose their rights and have no MLE.
Way too risky a proposition right now.