The existence of PMCs given contracts to perform difficult military missions proves simply that one doesn't need a standing military force to gain and retain the necessary skill sets to fight wars against adaptive enemies. In fact, Transformation's effort to make cookie cutter mini brigades rotating on a schedule between Fort Swampy and Hell makes the need for a force that can be stamped out of the ground essential for career stability at Fort Swampy. That was also the reason for the creation of military forces of the Dutch East India and English East India companies.
The abuses of Blackwater, Executive Solutions, and other PMCs in Iraq prove that these forces must not be under private control. The excesses of the East India Companies of the Netherlands and Great Britain forced those companies to be brought under national control for the same reasons. The bitterness of those excesses still lingers on and became the basis for more than one rebellion.
Now if someone were to think really deep, one could come up with a solution that could bring up the forces in configurations not previously envisioned AND be under control. How about creating a structure to do that? Maybe someone could call is the US Volunteers. Or maybe the Guard and Reserves.