I think it is the psychological impact on people that really cements for me that conscription really isn't sensible.
People who join the defence forces undergo quite rigorous psychological testing before they are accepted. That's not to say going to a war zone isn't going to affect them negatively, but there's a better chance they will be able to cope. They also have access to services to help them cope.
You wouldn't get that with conscription. Very sensitive people would be subjected to the culture of the forces and potentially to war zones. Complete nutcases would be given access to weapons. It's a recipe for disaster.
The whole concept is a popular thing for backbenchers to trot out though. The oldies love it (back in my day young people were disciplined blah blah) and it gets the member/senator more national - or at least State - media exposure than they are accustomed to.