I cant speak for everyone else here but the only reason why I have a res. is because I want some extra coolant in my system to compinsate for evaporation.
My system for the most part is a closed loop except for the res. where the coolant can expand and contract due to the different ambient temperatures. With this, I loose about 2oz of coolant every 2 weeks. The res. is there so if I leave my system alone for about a month, I wont be sucking air
Just cheap insurace.... no extra cooling potential.