Here's an old Skeptical CPA column where ex-Fedster Bob McTeer argues that it's just so mean to make the banks accurately mark the value of their assets on their balance sheets. If we just let everyone use fantasy valuations, everyone can be rich!
Mark-to-market isn't the problem. Excess leverage is the problem. If home buyers were required to put 20% down on legitimate appraisals, and banks had to keep reasonable leverage ratios like 10x on sound portfolios, bank capital could absorb the modest mark-to-market writedowns. And there wouldn't be big markdowns in the first place, because there wouldn't be giant asset bubbles periodically imploding.