Allowing a user to easily remove the battery is a beneficial feature.
In my opinion, built in batteries are a step in the wrong direction.
Some people do go on long flights/trips and having an extra battery to swap out is a life-saver for some.
Increased battery charge (as we all know) isn't always what it's hyped up to be.
Taking the ability to change out a battery from the user is not a welcome change.
What if the battery just plain goes bad?
Granted I haven't looked to see if Apple has altered its Warranty, but they have never covered batteries in the past.
They have always labeled them as 'consumables' and not eligible for warranty coverage.
If you had a bad battery in the past and Apple wouldn't cover the repair, you could purchase a third party one to replace it and save a ton of cash.
Now, you can't.
What about times when the computer locks up and the only way to unfreeze it is to cut all power to the machine?
I have had this situation many times with various Mac notebooks (PB G3s, PB G4s, iBooks, PB 1400s, etc) over the years.
If I wasn't able to kill all power by unplugging and removing the battery and do a HARD reset, I would have been in a jam and would have to wait for the OS to make up its mind and do something.
There are several other reasons why it's a bad idea, but I won't prattle on any longer.
Personally, I think the built in battery needs to go away.
I am in the market for a new notebook, but for now I will be staying away from any notebook that has an integrated battery.