With the Air also enclosed, I wouldn't be surprised to see the Macbook move in this direction also. Apple will say it is for aesthetic purposes, but really it is to get you upgrading your machines more often.
That said, changing the HD in the Air is a PAIN, but the Macbook Pro isn't that bad.
Perceived obsolescence and planned obsolescence. Many companies in many markets do things like this to get you to upgrade more frequently or pay them to have your computer serviced.
Ever wonder why you can't replace your iPod's battery? Well you can, but is it cheap to do, or easy? I guess thats subjective but you get the point.
Got to love capitalism eh