I think a good heater has to preheat the coolant. It may help to keep the manifold heated for intial cold starting, but that effect will soon be gone when the vaporizing fuel cools everything back down. On the other hand warm coolant means a warm long block and the thermal mass of all that warm iron will have a greater longer lasting benefit.
But like my gramma said just before she fell off her Harley, "Sonny, anything is better than nuthin'".
