I know it's a discussion that happens a lot with OSM. Many/most of these trails are unsanctioned and may contribute to erosion and other disruptions to natural habitats. Once they're documented on OSM, other sites that use OSM basemaps (e.g. Strava) pick them up, so more people travel them, and the impact multiplies. 

At this point, I haven't removed any of them, just untagged bikes from the ones that are only accessible from trails that prohibit bikes or lead to trails that prohibit bikes.