I do find it kind of silly, but I do see a point to it. For example, if I was to return to the game because I'm interested in the next expansion, I probably don't want to have to play through Cataclysm and Mists of Pandaria content just to get to the new stuff.

It also makes it easier to invite friends to the game. I've been in the situation a few times where one of my friends started playing the game and I had to spend the next month dragging them up through 90 levels of old content just so I can do current stuff with them.