The M9 has an interesting role in Iraq. You have to take yourself out of an American perspective where we have lots of pistols, deer rifles, shotguns. In Iraq there are a lot of AKs. It's not illeagal to own one and many households do. People there have grown up seeing light machine guns and they don't carry the psychological weight that they do here in the states. Also, remember that when Saddam's henchmen would execute someone in the streets, it was always done with a pistol. So, the pistol in Iraq means business. The pistol in Iraq is personal.
It's hard for the American mindset to understand but you can have a convoy with MK19s, M2s, M249s, and every Soldier with a M4. The convoy gets stopped due to a traffic blockage. People are starting to walk up to the convoy for handouts or whatever. You want them to stop. Do any of the previously mentioned weapons do the trick? Nope. Unholster an M9 and everyone stops. It's kinda crazy but in that theater, the M9 can be the most powerful weapon in a platoon. And you don't ever fire it.
All that said, the M9 is extremely valuable and effective in Iraq. However, it could be a Kahr, Sig, Glock, 1911, or whatever and do the same job. In the military if you are having to fire a sidearm, you've just found yourself in a very bad situation and you've been overrun.
Hope that brings some persepective.
Mark