Opportunity cost. If the options are a) extra payment on a debt at 5% APR, or b) invest the money at a > 5% after tax return, you would be better off investing the money. Especially accounting for the mortgage interest and student loan interest tax deductions.
I worked at a mortgage bank for a while, regularly saw applications from buyers with 7 figure net worths, could have easily paid cash outright, but they took on the 30 year fixed (jumbo/noncomforming!) mortgage debt. Why? Rates have been absurdly low, the interest deduction is a powerful incentive, and the cash that they might spend on purchasing a home outright, they would be better off investing.
On the other hand, each payment on a CC with a 22% APR is like a guaranteed 22% investment, which I couldn't imagine anyone advising against.
