Look at Jacobs Ladder, the section from the ford via the hairpin bend to the gate was once all grass and very ridable up (before bouncy forks), this section is mainly used by us MTB'ers and is now well worn because of over use
But really what difference does that make? The track is a few feet wider? A bit more difficult to ride?
Would we prefer the countryside to be a sort of museum piece that we can gaze at but not touch, or should we be using it?
Fair enough, protect the delicate, easily damaged areas (as highlighted by TJ), but why should we feel guilty about riding on the vast majority of trails.
Most areas that we ride in are not 'natural' in the strictest sense, they are all heavily influenced by human activity – whether that is Leith Hill, my local replanted slag heap, or even places like Dartmoor. We are just one of the newer users, and people resent us for that.
BTW I'm not advocating hooliganism on trails, I'm just puzzled when people invent crap to make me feel guilty for riding a bike.