The basic problem here is that all sorts of people do not have even the most basic understanding of mechanics.
If you want to talk about forces at all, then the student has got to have a proper understanding of the basic laws of motion. Otherwise the best that you can hope for is that they will remember individual 'sound bites' of your explanation.
Generally, instructors tend to assume knowledge.
The average person on the street does not understand friction, so their everyday experience tell them that when you stop pushing something, its slows down and stops. As a result, they don't really understand what a force is, and hence understanding Newton is just a no hoper.
The other problem is that straight and level flight is the obvious first thing to talk about. Sadly, its not the best way to do it. In straight and level, the standard 4 forces are conveniently 90 degrees apart. So the student gets the idea (subconsciously, even though you haven't said it) that all the forces are defined in those orientations.
I have found it useful to just draw introductory diagrams of different flight phases to show how the forces are defined (whilst stating any simplifications). Then you've got a basis for teaching S&L without causing miscomprehension.
Then, to teach the detail of different flight phases, you have to do the same sort of thing as the air exercise.
e.g. Climbing. To teach, you show student how to start, maintain, and finish. Basically 3 different exercises.
To teach the theory, you need to differentiate between steady flight paths, and changing flight paths. In the former, the forces must be IN balance, in the latter they must not. To enter climb, must have more lift to alter flight path. To maintain ..(bleh, can't be bothered to spell it out). To return to level flight, must reduce to level off, then increase back to match weight again to avoid descent.
But fundamentally, most students have forgotten (if they ever knew) their basic science, and the pre-flight briefing just isn't long enough to rectify that knowledge gap.
So basically I'm saying that if one is going to use force diagrams to teach, then I feel it is essential to get them right. On the other hand, what training value is actually derived from so doing?
In some ways I'm with B2N2 as far as the necessity of theory goes. But if someone wants to know, then get it right or leave well alone!
CPB