By sealing in the battery, you can have a larger battery since you won't be wasting space with the battery compartment frame.
So why does the iPhone 3GS has a smaller battery than almost every other smartphone with similar dimensions that have removable batteries?
iPhone 3GS: 1200 mAh (Smallest)
HTC Hero: 1350 mAh
Nexus One: 1400 mAh
HTC Incredible: 1500 mAh
HTC Evo: 1500 mAh
Motorola Droid: 1400 mAh
Blackberry Bold 9700: 1500 mAh
Oh, I know.
Design elements from the Macbook Pro don't apply to the iPhone. While not having a battery latch in a laptop allows it to have a larger battery, it doesn't apply to a phone. It doesn't make sense when someone talks about wanting the new iPhone to be "unibody" either. The iPhone doesn't have a frame in the first place; it's a shell.