Only works if everyone has to do it. Once you slice it into only a certain part of the population, it becomes an unfair class system. Example, rich and well connected people would never do it. If you say only men should do it, that's unfair as well.

Edit: I'm for this BTW, I think everyone should have to do some form of service, but I'd also like to see anyone in the gov with the power to send people to war be required to have served in the armed forces.