http://foldingforum.org/viewtopic.php?f=55&t=17730&start=45
That thread is a good one... basically, -bigadv was a decent bit slower on Windows than on Linux so it sounds like they chose Linux as the preferred platform.
Makes sense if F@H wants to get the "best bang for the buck" out of the WU's they send out. The speculation is also that they want more SMP units done period, hence the move to eight thread minimum for -bigadv in the near future. I would think you'd still be able to pull them off with your 2600k but with shorter deadlines you may get less points than the traditional SMP WUs would give you.
It does make some sense. If the GPU units gave too much ppd, too much community focus would move to the projects that supply them.
Since -bigadv were the "big fish" the ppd-valuing crunchers were going for it was likely unbalancing the super computer that is F@H towards getting them.
I, for one, think that it is nice that they have found a way to encourage both GPU and CPU crunching at the same time. Maybe it is just inherent to what F@H is doing, but on the BOINC side if I want some killer ppd GPU is the way to go and it makes donating the CPU cycles feel a little wasteful sometimes, even though the CPU approach is still needed for a majority of the projects.
Anyway, sorry to ramble on. Just my take on things with my relatively short time in the community