Think of the ideal-case scenario situation:
we have solid, proven defenders, in or just before their prime doing well, and playing in their best form.
Let's say healthy Andrade (in form), Chiellini terrorizing poeple on left....and for the sake of argument let's say we didn't lose Milito, and Ivanovic to other teams. Just stick with me. I think (and especially compared to present state of affairs) most of us would be quite content with this.
So: Ivanovic, Milito, Andrade, Chiellini as starters... not quite Thuram, Cannavaro, Zambrotta, (+ any upright mammal), in their prime, but solid.
If we were to possess a back line that is this capable (again, imagine), who would you want to retain out of the rest of the defenders we have now as backup, or rotation, or just in case of accidents?
If you think about it, Boumsong would not be one of them. And given we don't have the back line as outlined above, and he still cannot crack the line-up, sell him. His value is declining, and we get very little benefit from his presence on the team.
Given the present situation, and assuming (hoping; but not totally inconcievable) that Grygera gets adjusted to playing in Italy and regains form, getting Ujfalusi and Mellberg on free would be a decent step forward. Then we would not be under pressure, and we could actually attract one of the top defenders in the world eventually, and build gradually like we did with Cannavaro, Thuram and Zambrotta's development. This way, Mellberg, Grygera, and Ujfalusi would serve us very well for the time being, and could be supersubs once we strengthened the team later.
Following that plan, Criscito would probably be given time, space, and mentorship to develop properly. We wouldn't have to be worried about losing Zebina (to other teams, non-renewal of contracts, or injury). Molinaro's form would not be under such strict scrutiny, and Legrottaglie could live out his life as a positive role model player capable of stepping it up (but really just a backup for rotation players, unless he stays on fire like he is right now for the next three years which I really don't see coming).
Any way you look at it, and any contingency we might take as a team defense, Boumsong is not needed, and is not as good as any of the alternatives -- including the ones already on the team.