Sytes, although your linked individual masters degree thesis shows that horse traffic moves more sediment and especially when trails are wet (duh, who woulda thought), it does not really differentiate among effects of the other user groups. A previous thread about this issue pointed out that in Missoula and other areas there are mountain biking groups who do maintain trails, but I assert that there are significantly far more horseback groups and hiking groups who volunteer to work on trails.
My anecdotal experience with such trails used by all has resulted in a decision not to hike the Bridger Foothills Trail again. In a grueling backpack trip on that trail, the hiking was extremely difficult on stretches of trail which were trenched by bikes, motorized and non motorized. The ruts were deep with narrow flat bottoms impossible to hike, especially with a backpack, forcing one to try to hike on the shoulders or slopes above or below the trail. Horse and foot traffic does not produce those type of narrow ruts.
Admittedly now holding a bias, yet still pointing out the "wild" in what Wilderness does presently exist, I am adamantly opposed to wheels in the Wilderness!