Andrew Bacevich sees downsides to it:
Relieving citizens of any obligation to contribute to the country’s defense has allowed an immense gap to open up between the US military and American society. Here lies one explanation for Washington’s disturbing propensity to instigate unnecessary wars (like Iraq) and to persist in unwinnable ones (like Afghanistan). Some might hope that equipping women soldiers with assault rifles and allowing them to engage in close combat will reverse this trend. Don’t bet on it.
James Joyner counters:
[W]ith the dispassionate hindsight of history, the vast majority of America’s wars were unnecessary; whether we relied on volunteers or conscripts seems wholly unconnected to our propensity for bad wars. The difference, then, is one of choice: Anyone serving in Iraq or Afghanistan damn well volunteered to be there and was at least able to command a reasonable wage and premium benefits for their sacrifice.