I can't say I have the same issue. I do both (MTB and run). Our son is on a MTB racing team at his high school but won't run unless being chased by a wild animal. My wife is an avid marathoner but is still trying to get comfortable doing easy single tracks and probably will never be very excited about MTBing. But she is supportive of my time biking and doesn't view it as less valued than her runs. There is a bit of a difference in that runs tend to be training things rather than "outings". That's a subtle and subjective distinction though.
Beech - I think you need to keep explaining to your wife that she is just plain wrong on this point. You know how easy it is do that, right? :lol8:
BTW - I'm off to the MTB state championship this weekend for my son and his team!