I am quite surprised by the number of people here (and even larger numbers elsewhere this is being discussed) who don't seem to have any ethical problem with conscription. Thought the idea of killer slaves was completely off limits in the West. Now all of a sudden we are debating cost effectiveness. Guess I thought wrong.
None whatsoever.
Individual rights has real meaning only when others protect them.
Individual rights are never free.
You can have freedom of speech, movement, assembly and property rights only when the group you live in agrees that you have those things and protects them when you can't. When the existence of that groups existence or freedom to organize is threatened, it's time to pay retroactively for the rights you had and pay so that you can have them in the future.
In practice you can always serve unarmed or be in civil defense organization, but when whole nation prepares for self defense, everyone must make an effort.
I think professional armies are ethically more problematic. You create profession that fights and kills for your nation and government and it can be used to fight wars that are not that important or well thought. If war is not big national emergency, it's probably not a just war.
I think 'War Is a Racket' by Major General Smedley D Butler provides ethical solution. When there is a war, you conscript everything, people and profits. If one is not going to war, one works with the same pay as a conscript until the war is over. Business can't keep the profits until the war is over. This would take the ethical problems out from professional armies.