For what it's worth, the credit score system makes a lot more sense when you realize it's not about evaluating "this person's ability to repay debt", but rather "expected profit for lending this person money at interest".

Expected profit explains much behavior of credit card companies, but I don't think it helps at all with the behavior of the credit score system or mortgage lenders (Silas's example!). Nancy's answer looks much better to me (except her use of the word "also").