If your laptop webcam was a leap sensor, you'd be able to use it to control a virtual keyboard/mouse while resting your arm in pretty much anything around you. You can use your work table as a keyboard. Or if you're lying down, use your belly. Tho I'm sure that future leap-ready devices will have it's own built in solution (like a tablet case specially designed to fold forward to serve as support for using your hands to type or something). There's nothing inherit to the technology that implies you must move your hands more or hold them in the air any more than what you already do with current input devices. People only do so in the demos to show off the technology.
I mean, there's a lot of hard and interesting problems created by the leap that will be fun to try and solve. But a tired arm seems so simple to solve that it's even boring.
Unless I'm missing something and you're talking about something else entirely?