Bill Oddie wrote, "just ride the trails and don't worry about it. erosion adds character and often a good technical challenge to trails. i've never rode down a trail and thought "its too eroded, i can't ride it."
Depends, there's erosion and then there's singletrack that turns into swamps. Some trails really do benefit from wear and tear, I can think of a few but I reckon more are left worse for it.
Couple of people mentioned the pentlands, and that's a great example, there's some lovely wooded singletrack near my house which is currently a swamp, and which people still ride through even though there's a landrover trail alternative 10 feet away, and even though it's no fun at all when it's muddy. But the damage done in the next month will take half a year to recover- the mud will last longer, and the trail will be left wide enough to drive a car along in places. If I could put up "trail closed" signs in September and take them down in March, it'd be far better to ride for the remaining 6 months.
But, it depends where you are, not everyone has the luxury of avoiding vulnerable trails.