Matt Cutts writes here: [pbs.org...]
So why would they need such a function? Apparently they don't want to shake too much with an update. While some sites still may fall to nowhere, the big mass (the ordinary webmasters?) can relax, if they're near the top. OTOH, not too many can hope to reach the top from the bottom.
BTW: How do they measure the amount of churn? Do they collect "top X common queries"? But that would mean "non common queries" can still change alot, what is not what they want.