The problem with Big Al is he makes $15 mil. Assuming KG takes $10 mil to resign, that takes out all of our cap space, leaving us only with vet mins or the $2.5 exception to fill out the rest of the roster.
That would likely mean no Ray Allen (though that might be the case anyway) or Jeff Green and something like a Dooling/Daniels/Pietrus/JJJ/Stiemsma bench (in other words, a very weak bench).
I like Big Al, but I don't he's worth the price. Not enough of a difference maker.
Ray Allen's irrelevant to this discussion; he's gone, it's just a question of where to.
Big Al would help us MUCH more than Jeff Green would. Not sure how he wouldn't be a difference-maker; he adds an element to the team that was sorely lacking, and that Jeff Green doesn't. Jeff Green's just another body to help keep Pierce fresh; a healthy Pietrus can do that just fine, and even Sasha and Quis have looked OK in that role, at times. You can get a decent backup wing with the $2.5M exemption; you can't get a 20/10 power forward.
If trading for Big Al means Jeff Green walks... so be it.
EDIT: I should say, too, that if Jeff Green re-signs, it's probably going to be to be our starting PF. I'm kind of expecting to see a Rondo/Avery/Pierce/Green/KG full-time-smallball lineup next year. Not sure who else as a free agent PF would be any better - but we know Green's limitations at the 4. I'd feel much better subbing in Big Al to that lineup, even with his warts.
If not Ray Allen, whoever else we could sign with that money. The point is, if you cash all your chips on Big Al, there's nothing left to address our terrible bench.
There are several PF/C options on the free agent market who figure to be cheaper than Al (not just in cap space; they won't require picks to obtain).
Andrei Kirilenko, Carl Landry, Marcus Camby, potentially Sam Dalembert, potential amnesty candidates like Tyrus Thomas, Brendan Haywood and Al Harrington, Kwame Brown...
You could potentially get 3 of those guys and keep your draft pick for the price of Big Al - or two of those guys and a third perimeter player (Barbosa, Danny Green, Jamal Crawford, Jeff Green, JR Smith, etc.).
That said, I wouldn't be too bummed rolling out a line-up of Rondo/Bradley/Pierce/Big Al/KG. Al wouldn't give us much in the way of defense or team speed, but he'd provide a welcome offensive outlet. While his TS% is pretty underwhelming (doesn't get to the free throw line, doesn't hit mid-range Js at an elite rate even though he takes a lot of them), he practically never turns the ball over (2nd lowest TO% in the league), so he ends up pretty efficient. Combine that with his increased offensive versatility (a solid J, a million moves) and he'd make a very nice complement to the new Big 3 (Rondo/Pierce/KG).