Be it 100% stable or not a WHQL is a certified driver which has been tested...
But not tested for stability... and by stability i mean the way we think of it as users of our video cards. Like, playing a game does it do crazy things like overheat with wild swings in fan speeds, stutter, down-right crash to desk top or black screen, fail to launch certain games. That is, the hardware's stability whether it be the hardware itself or the way the driver coding interacts with the hardware.
The certification kit tests for things like how the install process proceeds, using security correctly, putting files in the right folders, updating registry all in ways compatible to the Windows operating system. Also, how the driver will interact with the OS after install. You could call some of that stability if done wrong, but it's not the kind of stability we're usually talking about.
Ultimately, I don't think it's right to see this as something Microsoft put in place to be some kind of last-ditch quality control mechanism to make sure hardware designers build good hardware and code device drivers efficiently and effectively. MS does it to make sure they don't bring down the teetering house of cards that Windows always seems to be and to advance a common look and feel to how the OS functions.
Also: where are you getting the BETA drivers from AMD? I only see release ones on their web site.