There is something to be said in favor of an all-voluntary military, but on the debit side there is this: only those with 'skin in the game' — either their own or that of their loved ones — properly appreciate the costs of foreign military interventions. I say that as a conservative, not a libertarian.
There is also this to consider: In the bad old days of the draft, people of different stations – to use a good old word that will not be allowed to fall into desuetude, leastways not on my watch — were forced to associate with one another — with some good effects. It is 'broadening' to mingle and have to get along with different sorts of people. And when the veteran of foreign wars returns and takes up a profession in, say, academe, he brings with him precious hard-won experience of all sorts of people in different lands in trying circumstances. He is then more likely to exhibit the sense of a Winston Churchill as opposed to the nonsense of a Ward Churchill.