Some mountain bikers and fatbikers are not following the rules. I wouldn't trust a rule allowing them in wilderness areas.
I live on the edge of public land in western Colorado. Yesterday evening I went for a 60 minute hike up Peak J (not real name) which sits right behind my house. The elevation gain to the peak is about 1,000 feet from my house. I can walk off my back porch, go about 100 yards, and I'm on a BLM trail to the summit. We had some stormy weather yesterday afternoon and the sunset last night was absolutely stunning.
However, I saw something else which brought my mind back to this thread: Several new pirated fatbike trails descending straight down from the peak.
I did a little off trail exploration and saw that fatbikers are quickly creating a number of new pirated trails. I did not see any fatbike trails on the mountain during my last hike, which was about 4 weeks ago. While following the new pirated trails last night, I came across an area where fresh sign showed a small herd of elk had been bedded (this is prime winter range). It looked like the bikers came through and busted the elk out of their beds.
Because the soil here at 7,000-8,000 feet is so loose, I am betting these new trails will cause significant erosion.
I just don't see a need to allow this sort of behavior in wilderness areas, and it will happen if mountain bikes in general are allowed. I'm a mountain biker, and my brother loves fatbikes, but there are good reasons for excluding them from wilderness.
I am in the North Fork Valley of Colorado. Any Rokslider is welcome to come hike with me and see what I'm talking about. It's a great training hike too.