Our latest, greatest slate of optimizations and fixes are always in the betas. The same is true of Google Chrome, Steam and many other programs we know and love. We iterate these beta drivers until we're happy with the codebase, then the final beta in that chain receives some last-minute polish and gets submitted to WLK testing for WHQL.
That last-minute polish carries forward into a new set of betas, along with all-new optimizations merged into the codebase from our developers for evaluation in and telemetry data from the field.
Beta drivers are always newer than WHQL, and the code is often quite a lot more mature than the "beta" tag suggests. I always, always, always run beta.
WHQL drivers are very important for OEMs, system integrators, your "average" user, Microsoft and other organizations that depend on a secure/WHQL/tested environment. They cannot afford the occasional hiccup of a beta, which is why we will continue to publish them when a current set of betas reaches a point where they're ready for release as WHQL.
//EDIT: Many corporations, for example, can only run WHQL drivers because their IT environments required Microsoft-signed drivers.
