I think it's more of an option for people who drives to places that has plenty of tunnels or underground or where light doesn't usually go. I see a logic in having them set to auto when driving in sunlight, then tunel, sunlight again and tunel again so saves you the hassle of having to turn them off and on all the time, though I drive with my headlights on anyway regardless of the sunlight
But yes I'd assume if you leave it on auto, as you have no need to turn on the car to have the headlights work, of course it will drain the battery when it gets dark. Just as bad as leaving your headlights on when parking.
I received mine today, and tested it with no results.
The connectors to the switch is different to the non auto light stalks. I will be making mods to my plug to fit it, there are two extra "rail" tabs sticking out the side in which the non-auto one has the groove for it to slide into. I'll update this weekend.