I'd always assumed there were two factors in choosing a pattern speed.
First, safety, thus the +5 for turbulence. The other was to place the
glider at best speed to fly. That way if you have to put the spoilers
away, you are guaranteed to cover the maximum distance. If I recall,
the simple formula for best speed was best l/d speed plus 1/2 the
headwind. Don't recall the second ever being explained though. Just
seemed to fit.