What's your definition of illiquid? Somebody offering $0.2 for a performing loan portfolio isn't functioning properly in my opinion. Yet banks are still forced to mark. Forcing an institution to take write-downs on performing portfolios they have no intention of selling just because the market is "worried" about real-estate isn't proper.
Arguing the real-estate bubble isn't a valid argument in this discussion since we are talking about performing loans. If the cash flows are there, it shouldn't matter what somebody "thinks" something is worth. Once those cash flows become compromised you have a valid point, mark it (eight dude).
My definition of illiquid is not what is important since I'm not the one that is making or enforcing the rules. I believe the rules (FASB 157) refer to "an orderly transaction between market participants at the measurement date.” From a practical aspect this means when there is a lack of market information, an entity is allowed to use its own assumptions, but the objective is still the same: what would be the current value in a sale to a willing buyer.
Credit risk is just one risk connected with a loan portfolio. Interest rate risk can be just as large of a risk, so just because a loan portfolio is "performing" doesn't mean that there aren't other factors that might require a substantial mark down. Also it should matter what the market "thinks" because the market is a less arbitrary judge of value than other sources.
I haven't said that mark-to-market is perfect. I realize that it isn't perfect. However, it has far less flaws than do the other options.