The only positive I see in the trend towards privatization of military contractor services is that it allows the military to effectively retain the services of highly trained personnel as opposed to losing them to the stateside private sector. A Navy Seal, for example, which costs hundreds of thousands of dollars to train, completes a hitch in the military and then goes to work back in the states. By allowing them to work as a private contractor through an independent business, we retain their services and experience. Of course, by allowing this, we give these same personnel added incentive to leave the military and earn more money as a contractor.
There are a number of negatives to this approach from my perspective. It is supposed to be cheaper. It isn't, or at least not in these two conflicts. What's more, military jurisdiction over these contractors is dubious in most circumstances, and if they kill civilians or engage a target they believe to be hostile, the rules of engagement and chain of command organized and adhered to in the military does not apply.
A major sticking point to US troops staying in Iraq past the December 31, 2011 deadline was the immunity of American troops and contractors, which Iraq has refused to grant. Such troops have rather routinely been involved in everything from human rights violations to corruption, so I could argue that the private contractors are also a political liability.