Keep in mind that not all sailplanes are designed for extremely light weight. Decreasing the weight also decreases the minimum sink rate, but doesn't help the still-air glide ratio.
Even in sailplanes that are designed for very light weight, designers have sometimes thought it worthwhile to add a rather large vertical fin. A good example of this is the Carbon Dragon.
The basic purpose of a vertical fin is to prevent the aircraft from flying sideways through the air, especially when the pilot makes an aileron input, which tends to create adverse yaw. Adverse yaw tends to be most pronounced in long-spanned, slow-flying aircraft.
Flying sideways is never efficient.
However, not all sailplane designers have always followed this philosophy. The Schiebe Bergfalke is an example of a glider with a rather small vertical fin. When flown with the pilot's feet off the rudder pedals, it tends to be extremely uncooperative in response to aileron roll inputs, being quite happy to fly sideways through the air rather than actually turning.