As someone that has opened up laptops before, I'd argue this point. With memory sockets, they can still run traces and possibly some low-profile IC's under the space where the memory resides, not to mention stacking memory modules vertically (while inefficient from a thickness standpoint, it saves surface area on the PCB). With the memory soldered to the board, the memory is taking up real-estate on the surface of the board. Last I checked (and I have a topcase for a rMBP sitting next to me awaiting swap tomorrow) the battery doesn't overlay the logic board, so gaining that extra vertical clearance does NOTHING for increased battery capacity.
This was strictly done to make the profile of the laptop slimmer and to improve reliability by removing a failure point (the sockets).