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.
And a fair number of people will just throw the controller away when the battery wears out instead of replacing the battery like they would if it had AAs. If you're talking about the lowest common denominator, there's going to be a lot of waste either way.
Plus, you can definitely swap out AAs in five years (or ten, or fifty). Whatever Li-Ion cell they use may or may not be easily available.