a lot of people run unit tests. but one thing to remember, is that testing is distributed, just like the development is.
An "-rc" kernel is basically the START of such distributed testing, not the end of it.
Many companies and individuals run test farms on -rc kernels (I work for Intel, and we do, I know other big companies including various distros do similar things).
Linus has a very strong "back out on regression" policy, but it's not a robot process, it's an "apply brain" process. Meaning, if the situation is well understood, and there's a clear bug, and the fix of the bug is less invasive than the revert... the bug tends to get fixed instead.