Wouldn't it be a waste of resources? Spending time and money on people who will feck off at the first opportunity doesn't seem like a good investment. If the "slave" labour makes up for it, then I'll concede the point.
The average day of a soldier not on deployment consists of marching, exercising, weapons cleaning, a few hours of advanced weapons training for specialists, more exercising, more cleaning. Then after 5-6pm if you're in the UK, you can piss around for a few hours before going to bed. That usually involves...well have a few drinks, chat, some psycho's do even more exercise, and then everyone's in bed by about 10pm max.
Put an 18 year old into this routine for a period of time, I don't see any significant cost of resources or harm to them. Might lower the obesity rate as well.
As for roles that a non-trained soldier can do, there are lots. If you have a HGV license (or the conscript is willing to take a course that gets you one) and the military equivalent, you can ferry supplies back and forth. If you speak a second language, you can provide second opinions on certain transcriptions and texts, if you have exceptional fitness you can be the lead pacemaker for certain long marches or runs. There's a lot of stuff they can do.
I hope to god the proposal isn't to actually send any of these people to combat regions or deployments though, that would be utterly insane.
But i'm against conscription/national service as a whole anyway. Statistically proven conscripts do not fight well.