I think they should do like in WW1, WW2, Korea, or Vietnam. If it is a war drop and better than what you have carry that instead. If all I had was a single shot nine, probably a corpsman on the front lines, then pick up a pppsh or AK and use it. COs should be banned from taking them away.
While I'm sure that many pickups from fallen enemies happen in serious firefights if one's own weapon is out of ammunition or nonfunctional, you would not want to use such on a continual basis, especially with enemy ammunition.
Logistics problems, the distinctive rattle of an AK possibly leading to a friendly fire incident, and the possibility of bad ammo destroying the gun in your hands would all be reasons to NOT encourage that, I think.
A lot of "war drops" were also anecdotal, such as the long-held stories that the British soldiers in the Falklands picked up and used the Argentine full-auto FALs instead of their own L1A1 FALs. It's simply not true, or was extremely isolated. Why would you use a weapon whose condition is unknown, and in a mode that makes it uncontrollable?