Mercenaries can provide several things to their employers:

1) Expertise. Mercenary soldiers may be better trained or more experienced than national forces. This probabably doesn't obtain in the US (with some possible exceptions).

2) Deniability. Mercenary soldiers may be hired in order to conduct military activities that the national forces do not wish to be held accountable for. This doesn't help the United States in our current conflicts - international media is more than capable of pinning a "contractor's" actions on the United States government. It's questionable whether mercenaries provide even a shred of deniabililty in the modern, information rich environment.

3) Expendability. National forces may be sensitive to their own casualties, but not to those of the mercenaries. The United States might make effective use of this trait, but for the fact that most of our private military companies are staffed by Americans. It was the gruesome murder several Blackwater employees that initiated the First Battle for Fallujah, for example.

4) Numbers. National forces may simply lack sufficient bodies to accomplish a mission and mercenary units can fill out the roster effectively. This is why the US employers PMCs in Iraq and Afghanistan - there is an extreme demand for infantry who can conduct security operations of many different sorts. This demand is so high that jobs that would ordinarily go to riflemen or MPs are outsourced at six figure salaries.

I'm afraid that as long as there is a shortage of effective foot soldiers, and mercenary units are available (both financially, legally and politically) then the US military will make use of their services.

A "healthier" use of mercenary formations would be places in South Korea, where forces are unlikely to see ground combat. Similarly, using contract soldiers to accomplish various non combat tasks, such as security and maintenance duties at US bases, might free up individual soldiers who could be re trained as infantry.