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.
Well the rechargeable AAs will wear out just like an internal battery, but there's more of them and they're individually packaged. It's a bit more waste and a bit more money, even if it's not a big difference.
Personally I think the big difference is in usability - I'd rather just leave the controller on a charger when not in use and never have to worry about swapping cells in and out. (I think battery degradation is overblown - it should last way more than 2 years, especially if you aren't gaming for 20 hours straight)