1) reduce the system stability (they're called "crap" for a reason)
2) give subpar performance/user-experience/functionality
So, if a driver is crap, and it is not doing any progress towards a higher, less crappy state of being, it will be removed.
It is in the rules of engagement for the staging tree, and it is a way to pressure people and manufacturers into improving crap drivers, before they go stale for too long, and decay into the "toxic waste" level.