This is just my opinion.. but I think a ref should be evaluated at least once a season and then should be given a raise for merit. By merit, I mean, not showing preference, by calling by the book, and being mentally and physically up with the game.
I feel that alot of refs side with their "favorite team, and/or coach" . Or feel that the homecourt advantage is a thing that they must follow.
I think the kids on the court, need a fair shake!
If this was in place, I feel the refs would have to be aware, and on their toes to do a good job.