Per Diem is a way for your employer to CHARGE YOU to generate your pay check. Per Diem pays you a daily non-taxable amount which seems great @ the time but has many nagatives.
1)Per Diem lowers your taxable income, thus it lowers what your remployer contributes (on your behalf) to Social Security. You inturn collect LESS SS when you retire.
2) Per Diem as was mention costs you up to .03cpm to enroll in.
3)Per Diem lowers your taxable income so IF you collect workamn's compensation for an injury you get less.