Quite apart from the fact that conscription is vile in and of itself, it's the creation of slavery to the state, we've good evidence that it's economically irrational as well. This little snippet from Iran illustrates the point:

Iran has been hit so hard that its government, looking for ways to fill a widening hole in its budget, is offering young men the option of buying their way out of an obligatory two years of military service. “We are on the eve of a major crisis,” an Iranian economist, Hossein Raghfar, told the Etemaad newspaper on Sunday. “The government needs money badly.”

The amount people will pay to avoid conscription is higher than the value the government places on having conscripts. Thus the loss to the individual must be higher than the gain to the government. This makes the idea economically irrational: it's a destruction of wealth.

Now of course we're most unlikely to have military conscription in the UK anytime soon,. But it's worth keeping that lesson in mind. And yes, this economic reality does also apply to all of the various plans floating around for compulsory "voluntary" service and all the rest. Enslaving people to the state is simply wrong in the first place. But the value that people put on avoiding it is greater than the value the government places upon their doing it. Therefore this is a destruction of wealth: really not what we want a government to be doing, is it?