Im not sure what the point is, either. It sounds like they want to encourage ( force? ) people in to playing in the expansions that they created for specific levels. It cant be for money because you already have to buy the expansion before you can advance into those levels. It sounds as if they have a preferred way they want people to play their game. This is contrary to the idea that the MMO is an open game which is built on a risk vs reward system.
Under their new rules, an 80 in Northrend will give you 10% the experience of an 80 in Cata despite how comprable their difficulty might be. I dont like it and it feels like they are trying to tell me how to play.