I haven't either - it's pretty good about automatically turning on and off, and I doubt the little leds in it draw any significant power.
Really like the Type 2, the keyboard anyway. The touchpad is pretty crap - I bought a Sculpt mouse which works just fine.
I'm one of those users - I fly 150k miles a year, international and domestic - and in my opinion, we're better served by an external battery like this one:
Microsoft Surface External Battery & Car Adapter Charger
For $130 ($145 if you want a car charger with it) you get a battery that adds 4-8...
Heavy, overpriced, doesn't sport the greatly improved Type 2 keyboard.....what's not to like? :)
I get 7-8 hours out of my SP2 as it is now. This cover would have been popular last year if it had launched with the SP1 but now it's a niche product for an already niche product.