I choose to look at it from 2 angles.
1. Would I want to have someone next to me fighting that does not believe in what they are fighting for? Maybe not, but in times of crisis, peoples attitudes about what they are doing change.
2. Ever since the draft was ended some 30 years ago, the numbers of elected representatives with any military experience has shrunk to almost zero. Without a well rounded character in civic responsibilities (one of those responsibilities is experience in the military that you are responsible for funding, directing, etc) we end up with an elitist class of politicians (note I did not say "representatives") that have no idea what they are hurting/helping by their actions that become laws.
I do favor some sort of mandatory service, if for no other reason, that will give a vast majority of US citizens a better appreciation for the country they live in.