Locally? Of course. Long term? Too hard to get ammo. Also the distinctive sound would be a problem in a firefight when pretty much everyone involved is firing at sounds. Firing an AK is just another way to get your guys to shoot at you.

Now special forces sometimes deliberately used AKs for that very reason. I read one account of a tactic a team would use to have one man with an AK. When breaking contact he would fire, and the VC/NVA would stop firing, thinking they were shooting their own guys.

Pick up as in exchange, or pick up in the heat of battle? Due to the kind of sounds the guns make, it'd be a tremendous risk to any soldier to pick up an enemies weapon and fire it. Friendly troops are going to fire towards where that sound is coming from, and you don't want that sound to be coming from you.

Plenty of GIs picked weapons aside from the M16 they were issued with, as they were allowed to, but in the case of them picking up AKs and SKS's to use for extended periods of time, it's pretty unlikely.