Would mountain biking help my road cycling?

I still can't quite figure it out.  I've gone to my share or mountain bike demo days and the legs/lungs don't seem to get a different workout.  It does make me wish that the road universe had an equivalent to the SRAM 1x11 drive train.  It also reinforces that I need to learn how to go downhill better.  I'm neither good at it on a road bike or a mountain bike.  The weird thing is that on a road bike I get nervous will all of the speed -- I get paranoid that my bike is going to slide out from under me.  On the mountain bike speed seems to be a good friend -- it's just about looking for weird things popping out of the ground (pesky roots and ruts).  I guess it worked by lungs a bit more but that wasn't based on cardio, just my nerves likely sending my heart rate up.