There's no doubt it's a "kludge" fix by Ford to save themselves from being sued by thousands of customers with cracked engine blocks. They knew the heater cores would plug up, so they put in that parallel branch to make sure there would always be flow even if the core was completely blocked.
I think the bypass hose was installed under a TSB
With my own two mitts, just finished replacing both Vulcan heads with rebuilds. If a feller looks at the coolant coursing through the motor, as a result of the pressure generated by the waterpump, flow is EVENLY distributed left and right upon exiting the pump, into each side of the front of the block, below each head. The coolant then EVENLY pressurizes each side of the block in the vicinity of the cylinder walls. This same coolant moves upward toward each head and is EVENLY restricted by the small holes in each head gasket. Once inside each head, coolant continues to pickup heat and move EVENLY towards the rear of each head, then via passages of EQUAL size up and into the lim
. Here the coolant flowing from each head joins up in the cavity behind the thermostat. An accurate temperature reading of coolant leaving each head at this time would be interesting. I'm betting on a healthy motor, the two temps would be mighty close.
The above presumes a decent flow of warming coolant by the thermostat, which will cause it to allow hot coolant to move to the radiator inlet. If no coolant is being bypassed back to the waterpump low pressure area, coolant temps in the area of the thermostat will seriously lag the need for thermostat opening. The hot area in the thermostat cavity is an ideal location to sense temps needed for the gages and computer. With the bypass presenting a good, hot, flow sample for the thermostat, place the temp sensors for the gage/computer in the same stream.
I have doubts about the Vulcan ever having a problem with even coolant flow throughout each side of he motor. Why the heater core itself was bypassed, was to guarantee the thermostat and the two temp sensors a failsafe working sample of max hot coolant. A plugged heater core would prevent this, without the bypass arrangement around the core. Quick and dirty fix.