Personally, I have no problem with leg extentions and curls as long as people are doing the bread and butter exercises as well. By bread and butter I mean some form of squat and/or leg press. Wish I could be more original with my mistake idea, but focusing on the "finishing" moves before you have the mass to sculpt is a dreadful mistake.
Bogdan Petia Sarac - Must keel moose and squirrel
Cancer Survivor - 7/21/10
Benchmark 5K time: 27:45 (3/5/11)
It's not the weight we move, but the people we move that matters. -- Bearded Beast of Duloc (12/31/10)