I guess I'm generally favorable to the idea, but with reservations, based on my experience with the draft.
The draft was good for:
- getting me out of a rut (different people have different ruts, but I suspect the recent decline in American geographical mobility is partly the result of the ending of the draft).
- exposing me to people from across the country and diverse backgrounds
- challenging me to endure and master new experiences: like basic training, like serving as an instructor.
Those benefits came because the draft was not voluntary. I'd worry that a non-military national service would not have the diversity nor the challenges. Once you allow the person to choose, you start to lose some of the necessary difficulty. Even in the Army, once I was past basic my cohort and co-workers were much more similar to me.
The other vulnerability of a new national service program would be, I think, the difficulty of finding a purpose to the program's work. While we draftees disliked the military, we knew it was important and/or significant. But we were essentially unskilled labor, cannon fodder, and weren't qualified for much more than that. And we got paid accordingly, so we were cheap. So what work requires cheap unskilled labor and is self-evidently important?
If the proponents can come up with an answer to that question, we can then talk about instituting "national service". Until then, we need more focused things like Job Corps and Americorps.