I've had hours long conversations on skype (2-3 hours). The battery can handle it. But Skype need not be "always running", it could be integrated as a service. So only the most basic components are running, using up as little memory and cycles as possible, JUST to listen for incoming calls. As soon as one is coming through it will launch the full Skype program.

You think your phone runs 24/7 just waiting for a call? When you have few resources you learn to economize. A cell nor a tablet is no different.