Great thread. I race a lot of MTB XC and Endurance races and the fitness helps on the ADV side a lot, but so does the skills. My coach (yes I have a cycling coach) has me practice drills on the MTB once a week.....things like high and low speed cornering, switchbacks, off-cambers, track stands, finding the best line on a trail and riding obstacles. This past weekend my MTB skills bailed me out in a very sandy section of "road", where i otherwise would have went down on the v-strom.
I've heard this same thing from top motocross and dakar guys.....they ride/race MTB and it makes a huge difference.
2013 DL650 ABS Adventure