Has anyone tried to cross-reference components from the Leaf's HVAC system with other vehicles they make? I'm very skeptical they did it the way they did just to save money on standard parts. Best I can figure, trying with limited success to parse the service manual's descriptions, is the water system provides an advantage of control over air temperature - especially if you pre-condition the vehicle before unplugging.
By having ~2 liters of good thermal mass you can provide very consistent air temperatures across varying air inlet temperatures. A bare heating element would have short but noticeable heat up and cool down times, but the coolant loop is a good buffer that will hold temp until the heating element can adjust. Of course, you sacrifice efficiency in the form of "standby losses" for a heating system you may never use.
I suspect the heater core is always at a temperature appropriate for whatever the desired cabin temp is, regardless of heating/cooling mode. Immediate air heating is controlled by the mixing dampers (5 & 6) while the water temp is adjusted on setpoint and other conditions. Even if you set the temp all the way down and turn the A/C off, best I can tell from the manual all that does is keep damper 5 open and 6 shut... the heater is probably still working.
Seems like a good mod to try and disable the heating element via switch to save a few watts.
=Smidge=