What if the laptop was hollow where the battery cells are. And what if something that looks similar to the Nokia 5165 battery would be put into this place. The battery would contain the same bag shaped cells.
you can't just expose a package like that to the user. user-removable batteries need to have a much sturdier (ie, thicker and heavier) package to prevent damage to the cells (major fire hazard). probably the chassis itself needs an additional layer of plastic to protect the components that would be exposed when the battery is removed. on top of all that, you need a robust latching mechanism that can withstand clumsy users removing and reinserting the battery over and over again. the end result is a much thicker and heavier device.
