If i was in Iraq I could pick up a ak-47 and ditch the m-16.
Ever heard an AK fire? It has a distinctive sound and that sound tends to attract bullets. While it might not be much of an issue in the day in a wide open desert, at night or in a dense forest(etc) anything that sounds like an enemy weapon often draws bullets. I wasn't there, I can't say from experiance but think for a moment.
You are humping through the jungle and you hear an AK blaring. You see the tracers and follow them back to the source(Or hone in on the sound, etc). But wait, one of your buddies has ditched his "POS M16" for an AK. It might be him... do you fire, risking that although it might be an enemy it could also be your buddy? Congratulations, you have just put your entire squad in danger by making them THINK about the situation. Thinking takes a second, maybe three or even five. A lot of things can happen in a few seconds.
Or in another scenario, you are carrying an AK in a night fight, during the Gulf War(I or II). You see a group of enemies and you open fire. There is another squad on a hill and they see those tracers(Or hear you, whatever) and assume it's an enemy. They light your arse up and perhaps your squad up too. It might not happen, it may never happen. But is it a risk you are willing to put on your squad?
As for the M16 being unreliable I have no experiance there. So no uncomment.