neat, I guess it has to be taken into account for chain length when replacing a chain (as the tension must bring the rd forward more than if it wasnt there)
Yes
is this a common feature on mtn bikes? I havent noticed this on bikes in stores but then I dont really look a lot at mtn bikes anyway. I could see it being a great help with fast downhill stuff on rough trails with lots of non pedalling rough sections.
It was designed for down hill racers to keep the chain around the chainring due to the extreme bounce down hill racers experience so you are correct in your observations but many casual MTB riders are seeing the benefit of the device and using it on there bikes now.
Cheers