Priority Tags do work providing only those who are entitled to them have them affixed - ie those travelling in First or Club only.
The problems of Priority tagged bags not being treated as priority arise when every Tom, Dick and Harriet has them attached. I have seen aeroplanes where there are twice as many "priority" bags as there are normal ones, so the normals got unloaded first!
Check in agents who are too weak to refuse to affix snob tags for those who aren't entitled are the ines responsible for spoiling what should be a wonderful service for our premium fare paying passengers.