The argument against stretching before training or a competition is that it may "dampen" muscle strength, and in some sports "loose joints" increase the risk of injuries.
A report in the journal Medicine & Science in Sports & Exercise, August 2008, says that a little easy stretching before training does not dampen muscle strength.
The Gray Iron interpretation is that you should save the more serious stretching for post-workout. That's when you should relax and take a little time to stretch out.
But before your training, it's best to warm up with movements that simulate your workout or competition. They should include enough range of motion movement to prepare your body and mind for what's to come.
Read more on the study here, and reach your own conclusions.