The other variable is how well controlled your heating is. A lower flow temperature means less overshoot of the target set point - and as losses scale linearly with temperature delta, that can mean higher energy losses (depending on the characteristics of the controller of course).
Whether or not you care about losses in unheated spaces depends on your system topology. Personally, all my heating pipes are within the thermal envelope of my house, so flow temperature has no bearing on those losses at all.
If you had a resistive electric boiler, flow temperature would have absolutely no effect on efficiency. You'd be completely right, that running heating only when you needed it would be more energy efficient.
