If it was user accessible like the last model unibody 13 inch MacBook, it would be a smaller capacity battery. Mostly due to needing to isolate the battery compartment from the rest of the machine to ensure people don't damage the mainboard. The casing on the battery would also likely need to be thicker to ensure it is not punctured easily while stored in a laptop bag, once again trimming back space that could be used for storing energy. Apple has their batteries custom engineered for their machines, and they pack them as full as they can. Most other manufacturers still use round cells, and just tie them together and put them in a plastic case. On their MacBook Pro page they even have a battery video to show the difference. It's likely a bit exaggerated in the diagram, but the point is still there that for Apple, a removable battery means wasted space and less power.