I just stopped by the site to browse and found this thread. There is so much poor information in this thread its comical. MTBers are notorious for being active trail building and maintenance participants--
1.this is because unsustainable trails are really unenjoyable to ride on a bike, ie grades over 10 percent, excessive roots and rocks from erosion, poorly designed switchbacks or no switchbacks.
2. It is well known, at least in colorado, that mtbers must (and do) yield to all other trail users. I always slow WAY down or dismount completely followed by a friendly greeting to alert the other user that I am approaching. 99% of the time everyone goes on their merry way. Its is almost always very easy to see hikers, dogs, and other bipeds on the trail LONG before you're 'on them'.
3. 20mph on singletrack in most cases is ridiculous, more like 10 or 15 max.
4. Mtbers shouldnt and often do not ride when the trail is muddy and loose for obvious reasons--slippery roots, no tread, and rutting. It is well known to any novice mtber that riding in mud conditions is not cool.
5. MTbers do not tend to enjoy riding 'off trail'. In over ten years of mtbing and backpacking I have maybe only once seen a mtber cut a switchback. In that same time I have seen NUMEROUS backpackers leave the trail, cut switchbacks, drop trash, build camps and firepits in plain sight, etc, etc.
I say ban morons from all trails.