Obviously there could be a time when a draft would be necessary as an emergency measure, but Carter and Glastris put forth a hideously unconvincing argument for regular mandatoryish service in the Washington Monthly.
Their punchline is that service (not just military) should be mandatory for anyone wishing to attend a 4 year college. The notion of course is to capture the youngins before they go off to college. But, this is rather classist view of college students. They aren't all fresh out of high school. Many are older/returning students. Many with families. This is true even in "eliter" institutions. Such a requirement would be an impossible barrier for older students.
Mandatory service is social engineering on a grand scale. Having gone to school with many Asian/Europeans who faced mandatory service at some point in their lives (or faced efforts to avoid it) I just do not see that as a good idea for its own sake. And, if we need more actual recruits, sweetening the deal and fighting fewer crappy pointless wars would probably solve the problem.
When the shit hits the fan, the draft will happen. But, mandatory service is a silly idea.