is as easy and smooth as possible

From a production standpoint, it makes a lot more sense to release such sweeping changes to the world before the expansion itself is released so we can ensure everything is running smoothly. We want to make sure the process of activating Cataclysm-exclusive content is as easy and smooth as possible.

This content spent a great deal of time in testing environments, but the best way to know things are wow po functioning correctly is to implement these data changes on all live realms. There actually is a lot of new content and things to see for players out there right now too (unless you prefer sitting in Dalaran at level 80 all night).

It's kind of win/win. You get a sizable chunk of Cataclysm content in a free content patch two weeks prior to launch, and we get two weeks to make sure any remaining kinks are ironed out. :)