No not really. People who tear up the trails or risk doing so becasue they don't want to shell out the $30 for a rear brake get dealt with as they deserve.

I am lukewarm to the whole idea of fg mtbing in general because it is far more likely to tear up the trails. What do you do on behind the saddle descents for instance? However the vast majority of the people who I have seen doing it are incredibly experienced riders who have the skills to do it well and have been riding the trails since thier inception so aren't doing much damage. Plus they could use the extra challenge. However I have little respect for others who do so before they have mastered the basic bike handling skills that are neccesssary to ride fgmtb without damaging the trails.

Not putting a back brake on is completely reprehensible no matter who you are(I have never seen an experienced MTBer do so only people who came from the street fixie side of things.) You often need to stop with both wheels while mtbing since there is so much less traction then on the road. A brake is much better at applying pressure to the wheel and therefore no matter how talented you are it will keep you from skidding at least sometimes. Skidding is never ever appropriate while MTBing ever. Since not having one forces you to skid more and tear up the trails more you are deserving of the aformentioned beating.