Comparing the ramifications on both sides of the floor is necessary and personally I feel you are understating the impact.
Ray Allen played well defensively in the playoffs and did a good job. He is a little undersized for my taste, but Ben Gordon is appreciably worse AND has a history of getting torched by the teams that we would be going against in the playoffs. Is that not a consideration?
I'd also argue that Allen also brings ball handling skills and finishing skills that are superior and necessary for a team that has Eddie House as their primary backup at the PG position.
Nocioni is a solid, albeit well overpaid for a very long time, 6th man.
Allen did well defensively against Kobe and that's it. Joe Johnson repeatedly took it to him in Atlanta's wins and Rip Hamilton was easily the Pistons' best player in the ECF. This has nothing to do with his offensive struggles either, he's not a good defender. He's absolutely better than Ben Gordon, but it isn't like we're downgrading from a position of strength to a position of weakness, we're going from bad to worse. I'm fully aware of the impact there, but I think you're overestimating Allen's defensive ability.
Nocioni is a
significant upgrade at reserve swingman, where we currently either have TA playing out of position(he's smaller than Allen) or Giddens(undersized, out of position) or Bill Walker(coming back from another knee injury) playing as untested and unheralded rookies. My contention is that the impact of going from Allen to Gordon(who should be able to do a reasonable impression of Allen) in the starting lineup will be outweighed by the benefit of adding Nocioni to the bench to plug a significant hole.
I'm going to point to no rumors about Ray Allen being traded (outside of this board) as the Celtics having no interest in trading Allen. Last time I checked, the Bulls ended up with the #1 pick and the Celtics ended up with the last pick.
I'll state it again. Ben Gordon does not have the value of Ray Allen and I would not do a deal with him as the principal part of the deal regardless of whether they deal Deng or not.
Ainge is on the record somewhere(admittedly, I forget exactly where) saying that he isn't going to repeat the mistakes of the 80's team, where the core was held onto for far too long, ensuring the franchise had to go through complete rebuilding. Garnett and Pierce are the only irreplaceable players on the roster, and the two guys the teams' truly built around. Seeing as Allen is going to soon be a (hopefully) productive big name on an expiring deal very soon, it's only logical to conclude that Danny will field offers for him in order to restock the team and extend our window for championships.
This isn't a deal built to get Ben Gordon, it's a deal to improve the team. Gordon should be able to slide into Allen's role in the starting lineup very well, while Nocioni vastly improves our bench. Also, and very significantly, this move would still leave us half the mid-level and a roster spot to pursue a veteran big man, be it Mourning, Mutombo, or whoever else Danny likes. We currently have two major holes, backup 3 and veteran big man, with only a little money and one roster spot.