So they could have a bigger battery in there.
I have over 40 laptop-users under my wing. And not one of them have several batteries that they switch when one battery runs out. Not one.
And why would people want to switch batteries? Because the existing battery runs out of power, correct? But if they can put a bigger battery in there, the need to replace the battery goes down.
I bet that Apple has done extensive market-studies, and they discovered that only about 1% of laptop-users switch batteries.
They can make a bigger battery while keeping it user-removable. The two aren't mutually exclusive. I'm pretty sure Dell and HP have machines that have the ability to go up to 24hrs.