The instructor has a multitude of tools available to get the lesson across in the most practical and effective way possible for the student. A PP presentation is just one of those tools as with models, aircraft parts, a whiteboard, pencil and paper, practical demonstration, flying hands, etc.
Nothing wrong with designing your own PP presentation, and then tuck it away never to be used in anger. But to my mind, I'd rather improve on something aleady out there and pass it on to the next person to do the same. A pox on the instructor who reads from a PP presentation without uinderstanding the subject! A PP presentation should supplement the lesson, not BE the lesson (most times).
Tough to beat the flying hands and models of a passionate instructor who knows what he/she is talking about.
What works better: showing air flowing over a wing on PP, or doing it yourself with a model? Quicker on PP, but the message is stronger from the practical experience. A little of both would be ideal.