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.
Batteries for PS3 Dualshock are still available on ebay in my country. You just unplug a cable and remove a couple of screws. AA are not worth it.
AA should only used for flashlights (emergency), kids toys (safety) or when you are far away from an electricity network for much time (days, weeks).
Rechargeable AA work just fine and are easy to acquire. What exactly makes them "not worth it".
Your controller stays mostly in your house and can be charged with a cable. No need for recharging devices, thinking if you have stored charged batteries, it can play while charging.
And if 20 years later you can find a replace battery for a ps3 controller that means your device probably can live for 30 years or more.
What benefits bring the rechargeable AA batteries for that particular device?
You don't have to worry if there will be a battery available in 20 years because AAs are standardized and will always be made. Or at least far more likely than whatever proprietary crap they're putting in these controllers. Devices that take AAs will last until the hardware wears out. You don't have to worry about the battery being the weak link. Which has been the case for every rechargeable device I've ever owned. The battery is always the first to go.