So what if those 'kinda interested' guys turn into 'really interested' ones after they learn to fly? You're advocating turning away guys who are not entirely sure that RC flying is for them, which is probably around 50% of the guys that I instruct.

And for the 'motivating them to learn because they're paying for the time', there are many people who are very motivated but are going to take longer to learn than others. That motivation will turn into frustration if the person is taking longer than average.