can understand on the driveway pictured it would be a good idea to lay 2 concrete tracks as it looks like the parking is restricted by the width of the drive on that side and gets heavy use,but if the base is done correctly ie taken down to clay/shingle etc depending on the area and conditions where you live there is no need for concrete you still need a screed coat of sand, its when people use to much sand caused by not leveling and preparing the base properly that causes problems.
on some applications ive known some mix cement in with the screed coat,but properly whacked type1 and type 2 crushed stone is more than enough if done properly,dont use dolly personally as it absorbs water and goes spongy,but like everything else different people do things different ways