(Wow, what an interesting debate. No one has thought of this before.)
Correct, the sidearm is ALWAYS backup. WHICH MEANS, if I am ever in a position where I HAD to use it RIGHT NOW, it means everything else has failed. You are in more trouble than you can handle. THEREFORE, you need the sidearm and ammo most likely to do the most damage. If the military would quit pretending to follow the Hague Accords, and issue effective ammo for the M-9, I would still PREFER the 1911, but I would shut up and live with 46 rounds of hydra-shoks.
I'm not seriously picky about high capacity. If you are in a situation that can't be solved with 7, 8, or 10 +1, I don't see how 15 +1 is going to make a whole lot of difference. If it were absolutely critical, one could use a wide-frame 1911.
If I went to Afghanistan, and wound up attached to a unit with civilian clothes and vague identities, I would ask the CO if he cared what I carry. If he told me he could care less, I would immediately have my 1911 shipped in pieces and carry it, even if it might mean I would have to leave it there, since getting one is easier than taking it home. (I already know what the liklihood of this is, keep it to yourself.)