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, and actually hurts the glide ratio against a headwind.

Even in sailplanes that are designed for very light weight, designers have sometimes thought it worthwhile to employ a rather large vertical fin.  A good example of this is the <a href="https://en.wikipedia.org/wiki/Maupin_Carbon_Dragon">Carbon Dragon</a>.

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 <a href="https://www.av8n.com/how/htm/yaw.html#sec-adverse-yaw">adverse yaw</a>.  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 <a href="https://en.wikipedia.org/wiki/Scheibe_Bergfalke">Schiebe Bergfalke</a> 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 inputs, being quite happy to fly sideways through the air rather than actually turning.

Certainly the purpose of the long tail boom and large vertical fin is not to "balance" the weight of the cockpit.  Rather, the cockpit is placed in the optimum position to balance everything else.