Based on the description on their site, the controller includes a built-in battery: "8.39 Wh Li-ion battery, 35+ hours of gameplay... "
That was disappointing for me. Specially condidering the Steam Frame's controllers make use of AA batteries: "One replaceable AA battery per controller, 40hr battery life"
AA Batteries might not be as convenient to use, but being able to replace them is a great advantage. All my Xbox360 controllers still work fine, but none of my PS3' Dualshock 3s.
The official docking station could be used to recharge (rechargables) AA batteries so the functionality could remain the same.
Because I don't want to be tethered to a cable.
Because then I have to be constantly concerned about the state of the battery at any time. I have enough rechargeable devices to be worried about.
Just have the magnetic puck exactly in the location where you leave your controller when not playing and done, never think about the battery and don't swap things.