Another problem with these types of studies is what they don't say.
Who did they study? In shape 20 somethings that excercise regulary?
Out of shape 30 year olds who sit at a desk for 8 hours a day and go to 2 X 1 hour classes a week?
What type of excercise?
The needs for pre stretching in weight lifting or running are quite different than that of martial arts.
Also what do they mean by stretching?
A 30 minute intensive yoga like deep stretching regime or 10 minutes of limbering up.
I agree with Alex the Karate teacher in the OP is full of crap and has taken what he was told out of context.
As is the case with almost everything the answer is moderation. A moderate warm up / stretching regime to get yourself limber is always a good thing.
Try telling ballet dancers not to stretch before a perfomance and they'll tell you that your retarded.