Using your argument about checking the C150s undercarriage (even though its fixed) why don't you also train them to turn the C150s fuel pump on/off - even though it doesn't have one.
As for your airline analogy - I hardly know where to begin.
Fly the thing you're flying! If its got a retract U/C, fuel pump, cowl flaps etc etc use them as required. If it doesn't - well, it doesn't.