The main drain has value other than draining out the pool. The water in the pool will mix better if you have it coming from the deep end as well as the pool surface. The chemicals will mix better, and probably since cold water sinks, it will also serve to better regulate your pool temps. I usually keep my valve in position to have the skimmers suctioning about 2/3 of the water, and the main drain about 1/3. If I shock my pool, which I do by broadcasting powder over the deep end, I'll turn the valve to the middle position to get more of the shock into circulation. If I have alot of garbage floating on the pool surface, I'll set the valve to only the skimmers for a few hours. But my usual every day setting is 2/3 skimmer, 1/3 main drain. Hope this helps.

Tom