Performing a rollback on something as complicated as an operating system is an imperfect process. I wouldn't provide an option for it either. If you need to roll it back, you do so by restoring from a backup.
I've worked on Windows PC's with failing RAM that would BSOD during Windows update (to a new build) and they still could manage to roll back to a previous version and maintain stability.
Windows makes a duplicate of the Windows/System32 folder and can recover from a total crash or rollback to a previous version if the user requests it, and it works perfectly.
There is no excuse as to why Apple can't offer a similar solution.