I use it for the same reasons Lee cited (although, not for 20-years! Lee- I hope they give you a long-term client discount!).
Two additional thoughts:
How many appraisers who "cite" M&S as a source on their reports actually have it? Do you think maybe 20%? I doubt if it is more than 5%. I suspect that every state regulatory board or any attorney with experience in litigating against a residential appraiser believes the chances are much better than "even" that a report that cites M&S as a source didn't use it in the analysis. I cite it and use it, and keep the back-up in my workfile. If I'm ever questioned, at least I'll have additional piece of evidence that will add to the credibility of my reporting process.
The new Fannie form states that the Cost Approach data must be cited so that it can be replicated by the lender. So, I cite the version and update of my M&S (Res. Est. 7, 03/06), the quality/condition rating, local multiplier, "current cost" factor, and any additional adjustments I make to the cost figures. A lot easier than citing a local builder or contractor with Tel# & contact info (which I've never read in any of the reports I've reviewed, even though many of the reports say "per local builders") as my source.
In my mind, as costs go, this is relatively "cheap" compliance insurance.