Cat, my m60s have rear ports and I have my entire front wall covered with absorbing material. It works great since you are still reflecting some sound off of the back of the screen and you need to "kill it" before it just keeps bouncing around between the wall and the screen making the sound muddied.
Plus, the absorption is for more of the upper frequencies (except the bass traps, but those aren't right behind the speakers) and the sound coming out of the ports would tend to be more lower frequencies (I know, these are smaller bookshelf sized speakers, and not deep bass producers) and those sounds won't be absorbed by the dampening material.
While your logic seems solid, and until I actually did it, I would have tended to agree with it, but in practice it is OK to put some absorption behind these speakers. Now, LFR speakers would be a different story, but I would not be sure that they would be good for behind a screen wall anyway.
M60s, VP180, VP150, QS8s, SVS 20-39PCi, HTPC, JVC RS45, Onkyo TX-NR709, Shakers