Seems to me that the tanks starting coming out on OEM's right around the mid to late '60's, and they were quickly seen in "universal" kits for retrofit to the older cars. Though they may not be as critical for a vertical core, I believe that they increase the overall heat capacity of the system by increasing the volume of coolant circulated; and they also eliminate the coolant/air interface in the tank to help reduce corrosion in the engine. IMO they're a good thing, necessary or not.