I'm not sure I understand what it is you are trying to make clear here Guppy: Light aircraft - yes they usually have piston engines, but does it make any difference to the carb whether thay are of the hoizontally opposed variety? Suerly the carb doesn't know what piston line up the manifold leads to?
Yes, it really does make a difference. Most light airplanes using horizontally opposed powerplants today don't come with carb air temp gauges, for one thing. Most use updraft float carburetors. Light singles using various radial engines often use pressure carburetion of a downdraft variety through a supercharger, and often do have carb air temp instrumentation, and different requirements and application procedures and techniques. A 1340 or R985 isn't operated the same as an O-360.
The techniques and practices for a given airplane aren't universal, either. One uses carburetor heat where needed, as a tool for engine operation. To blindly apply full carb heat at a given phase of flight is doing little more than taking an ignorant, wild shot in the dark...and doing little if anything to address the issue.
Should carb heat be applied blindly on the downwind then shut off? It's appropriate some times, and not others. Should it be left on until power is applied at the end of an approach to landing? Sometimes appropriate, sometimes not. It's a tool to be used, and it's use varies according to the conditions, installation, appplication, and even operator experience, to say nothing of instrumentation available.