Without overflow reservoir likes in cars of the 60's and early 70's there is some air in the top of the radiator when the engine is cold, but no air in the engine or in heater core when the engine is running. The non-pressured reservoir eliminates that, but the main advantage is you can check the coolant level anytime without removing the radiator cap.
I just changed coolant in my Honda S2000 few months ago, after flushed with distilled water once and drain most if not all of it, I just pour in a gallon concentrate Peak Global Lifetime then top off the radiator with distilled water and filled the reservoir with distill water only. The capacity is 7.8 quarts(reservoir included) so 1 gallon concentrate makes the coolant at 51-52%. I didn't have any problem with air in the system, probably when I drained the old coolant, flush with distilled water and filled with new antifreeze I set the temperature at max hot, this let the coolant flow through the system freely. Also, I park the car up the fairly steep incline driveway so that the top of radiator is the highest point, and loosen the bleed nipple to let the air escape.
