Which Sebastian Mallaby points out in his column today:
McCain's plan could exacerbate the financial crisis in a perverse way. Help for families who are behind in their mortgage payments could encourage others to stop paying, too, in which case loans that are now good would quickly turn rotten.
That doesn't mean you don't help families struggling to pay their mortgages. But you've got to be more careful than the crude "buy up all the bad mortgages" approach McCain is advocating. You've got to have fairly rigorous criteria for who qualifies for help, which the McCain plan seems to lack.