How important is a streamline to a swimmer? I think it can make a world of difference, particularly in a short-course setting, since streamlines and the idea of reducing drag can be applied to starts, swimming, and turns. My take on swimming and streamlines is they are a way to make a minimum energy investment for a maximum speed return. Not exactly a free lunch for a swimmer, but as close as it gets in the swimming pool. Initiating the first kick or pull is a matter of determining when the swimmer's speed is about to drop from faster than they can swim to their race swimming speed. Experiment with different timing
From a start, the speed gained from the push off of the block and from the force of gravity is faster than the swimmer can actually swim. If they can maintain that speed for any extra duration, and everything else is equal, their overall time for the race could be quicker. And all they had to was perform an improved streamline.
During the swimming portions of a race, any chances to reduce the external forces fighting against the swimmer's forward progress (like drag) can result in a faster race time. If a better body position through a slight adjustment of head position results in decreased drag, then the swimmer just got faster - without putting any real extra effort into moving forward any faster. Other ways to reduce drag include paying attention to hand entry and hand/arm position (both arms!) during the stroke cycle. And don't forget the legs. A wide kick might have more force to it for some swimmers, but it also increase drag, and it is likely that the wide kick's force is working to overcome the drag it creates, resulting in little or no added speed (in other words, a narrow kick could be more efficient).
What about turns? Lot's of chances to reduce drag on those things, open or flip. How is the direction being changed? Is there a loose limb sticking out someplace that is being "dragged" through the water instead of slipped through it? Is water being pushed against or slid through during the direction change? How about the swimmer's push off the pool wall. The swimmer's upper body must be in a streamline shape prior to the initiation of the push to maximize speed off of the wall. As the push-off continues, the swimmer must pull the rest of their body into a streamline so they area able to hold that speed (which should be faster than swimming) for as long as possible.