Do Ratios make sense for Underwriting?

A former colleague of mine (who is not on the record) at Freddie Mac says that using ratios to do mortgage underwriting makes little sense; that lenders should focus on residual income after debt service instead.

Consider two households, one of which earns $30,000, the other of which earns $100,000. If the first household pays 25 percent of its income for housing, it has only $22,500 gross income left over for everything else. Of the $100,000 household pays 30 percent of its income for housing, it has $70,000 of gross income left over for everything else. Clearly, the first households is under more financial stress, and is at greater risk for delinquency than the second. And yet lenders persist in using ratios to determine who gets mortgages and who doesn't.