I think when a lot of trainers start focusing on speed, their form accidentally loosens a bit, increasing the chances for injury. So obviously from a safety angle, you should always strive to be tight and controlled, no matter what your speed.
From a muscle stimulation/hypertophy angle, you can create muscle stress from both sides of the speed spectrum. Going slow prolongs the time a muscle is working (T.U.T.), while going fast, or explosively, creates a higher degree of actual perceived load for the shorter duration of the movement. Both approaches have validity.