Originally Posted by Retro Grouch
If you do real mountain biking, you are quite likely to bash your rear derailleur out of alignment sometime so you are more likely to need one.
If you do real mountain biking the thing will end up breaking into little bits all by itself and then you've just inadvertantly become a trail litterer. I cut mine off with a pair of plant pruning snips (shhh... don't tell my wife). Save yourself the trouble of picking up bits of plastic from the trail and just cut the thing off now.