Historically, "volunteer" armed forces were mercenary forces. In a way, current "voluntary" national armed forces are essentially mercenary forces with exclusive contract. The current volunteers are paid regularly like mercenaries (they even have pension plans) of prior eras, they are limited in number in comparison to the general population, and for some, the field is essentially a career. Unlike the mercenaries of prior eras, the modern volunteer army lacks the freedom of association and is legally bound to serve only one state, even if that state is not the best benefactor.

Mercenary, or volunteer, force is useful to a sovereign (or parliament) because its use is not subject to much public outcry. After all, most of the public is not affected in the use of force against another state, excepting for loss in monetary resources. The sovereign does not need to grant as much concessions to his subjects, when using a mercenary force, since the burden of war usually falls on the state coffers, rather than lives of subjects lost. This lack of public accountability allows for a state to conduct more ill-advised foreign adventures than when using a general conscript army, since the public is disinterested. The ease at which the US sent her volunteer army into ill-planned and purposeless wars in Central America and Middle East over the last generation displays the degree of public indifference when using a mercenary force to conduct wars.

I think modern volunteer army offers all the negatives of a conscript army without any benefits of a mercenary force. Modern volunteer force is much like the pre-French Revolutionary army of the Monarchs - conscripted from the lower strata of society, given cohesion through iron discipline and fear of the officers' whips, supplied by the Royal treasury. The only difference is that in the modern army, the members are enticed by financial reward and convinced by drivel of nationalism to join a sub-par mercenary outfit.