People join for many different reasons.  Probably one of the most common IS because of lack of options, or money for college.  I joined because I was a deliquent, and didn't have much of a choice.  It was one of the best decisions I made.  I went SOS, got money for college (although not much back then), learned discipline/maturity/responsibility, and used my experience to better myself.
As a result, I would enthusiastically vote for a mandatory 2 years in service.  I think it would be a favorable experience for every American.