Diesel engines make little heat idling in frigid temperatures. Not near enough to heat the passenger compartment sufficiently. Northern districts will install an espar or webasto coolant heater in order to create additional heat in the cooling system to sufficiently heat the passenger compartment.
The stock heaters can move a large amount of BTU's, so they're really good at putting out heat in super cold temps. However, most skoolies avoid those temps, and their conversions include removing the passenger heaters because they're not needed, in the way and take up precious floor space. If you're not going to be in the cold, there really isn't a need for them or your espar heater. If you're in the frigid cold, you can design your bus around the stock heaters so that stuff is all utilized. If you won't be in the frigid cold, you'll likely be better off using a different source for heating your bus, be it an diesel air heater, propane furnace, or a heat pump.
It all depends on your usage and climate.