Terror, unrest, and total pandemonium are the classic beginnings of any good adventure flick - but they're not what you want to see in every day life. If the United States instated a military draft, you'd be looking at all of those things (and worse) on a daily basis. As Americans, we are used to all of the wonderful freedoms that we enjoy - and most wouldn't take that kind of order lying down.
Everyone who is serving in our military today chose to do so. No one told me that I had to sign up, "or else." (However, recruiters can be quite persuasive.) Forcing military service on the citizens of this nation would be a grave mistake - not only because it would disrupt lives and cause civil unrest, but because the military doesn't want people who didn't choose to be there.
Americans know whether or not military service is the right choice for them. The citizens who balk at being drafted would cause so much fuss about it that the general population would flip out, and all Hell would break loose.
In addition to the panic and general freaking out that a draft would cause the citizens, it would weaken our military exponentially. There are many factors involved in this theory:
They'll be unhappy, which will drag down morale in their unit.
They'll be hard-pressed to do their job, because they just don't like it.
They won't be able to function well with the other members of their team, because they don't agree with what they're doing in the first place.
Everyone's familiar with the old saying, "You're only as strong as your weakest link." That's not to say that those members are weak, but it does mean that the military will have a hard time functioning as a cohesive unit (which is a necessity in battle), thus becoming weaker.
It's difficult to imagine if you haven't ever served in the military, but try to picture this: you've been deployed to Iraq or Afghanistan, and you're in the heat of a firefight with some really determined insurgents. The guy who never wanted to serve in the military in the first place is suddenly a serious mess - his training has gone out the window (he didn't pay much attention while he was being taught military skills, because he was focused on ways to get out of compulsory service) and he's panicked and not returning fire.
You and the others with you who have been in the military for a while - and who are well-trained - are doing what you can, but it's difficult without everyone in your team helping. You not only have to pick up the extra slack, but you've got to protect the guy who won't protect himself.
When it's all said and done, the guy who didn't want to be there is now suffering from psychological trauma, and he's no longer useful as a Soldier. They've got to send him home, and the military has just wasted tens of thousands of dollars on his training, deployment, and psychiatrist bills - and they've changed his life for the worse, negatively affecting his family, friends, and even his job.
It's easy to see why a draft would be a waste, especially since we have a sufficient number of volunteer service members.
That said, I'd rather have a volunteer guarding my back any day.