This argument that these assets have inherent value is silly, if they had inherent value people would want to purchase them.
i respectfully disagree. the inherent value is the future cash flows of the underlying securities.....note that while the default rate on residential mortgages is some where around 4-5% (i believe)....so this suggests that roughly 95% of Americans are paying their mortgages in a timely manner. Now, while no one may be willing to buy collateralized security instruments on the open market due to a lack of transparency, or increased uncertainty, or an inability to leverage the assets, or a number of other possibilities.....the value of the cash flows still exists.
a more appropriate valuation technique for accounting purposes would be a mark to model...you derive a net present value based on a series of assumptions. Those assumptions must be well founded & stress tested but this technique will provide a more accurate pricing mechanism than marking to a nonexistent market. in fact, the mark to model is already in place for some assets where market inputs are not readily available.

Hiya Goofer--haven't seen you around campus lately.