-Braking-induced skids are useless.  Skidding reduces your stopping power 
 and increases your stopping distance.  Also, you have no control over the 
 wheel that is sliding.
-Turning-induced skids are useless as well.  If you are skidding, that 
 means you are riding too far on the edge.  There are ways to save a 
 turning skid, but I'm not going to put it on the FAQ since skidding is 
 not recommended.
-All skids tend to destroy trails.  If you skid a lot on your local 
 trail and it's closed after a winter, you might be the cause.