Your instincts are correct. But the builds do cover different aspects of the OS, and are released in order to test those particular items for public reaction. There is also another avenue of release, to which most are not privy. That is through (selected) partners, mainly those directly involved in the IT business, and, in particular, those engaged in OEM retailing. The latter are the most important, imho, as they can see if the product, so far, is able to run on specific machines. The former, however, are the weak point, again imho. They have to ensure that the hardware is up to date, and specifically modify drivers and other software, before RTM. They are crucial to the acceptance of the final product by the public. This group were very much responsible for the bad reputation of VISTA. They saw the new OS as an ongoing product, whereas it was quite a big change from previous releases, and failed to come up with corrected software..
I imagine that the "slow" builds are the efforts made to get the pieces to work together, and the fast ring is where you take your chances on a build put out for reaction. We were warned!
At this stage, I would imagine that MS have all the feedback they need, and have now got to a stage where they need to apply themselves to the significant problems and get it all to run together.
In view of that, I am hoping that, from now, we are going to see some stable builds in the safe ring.