Sean McFate writes about
mercenary military forces for
The New York Times. As he notes, mercs are cheaper.
Mercenaries are less expensive than standing armies, just like renting a car is cheaper than owning one. (snip) Blackwater cost 10 percent less than a comparable army unit in wartime Iraq, and a private force costs nothing in peacetime because its contract can be terminated.
When not employed doing something good, unemployed professional soldiers sometime find work doing something not-so-good. They have been recruited to overthrow third world governments, or guard drug kingpins, for example.