I've read a few articles about that. I think it's rubbish saying you don't have to stretch before a workout. Every time I've tried it I've ended up with a really bad backache afterwards and pain in my hips to boot. I actually know many instructors who don't stretch as part of the warm up. Many people seem OK with it but I haven't seen any of these instructors produce any students with any impressive amount of flexibility. I've found that stretching too much before working out can make the legs in particular feel uncoordinated and sluggish but a certain amount of stretching is very beneficial to the workout.