I would say that the point of shifting gears is to stay in a comfortable and efficient cadence as the terrain changes, not necessarily to go faster. I've never measured cadence, but I have a feeling that once I'm up to "cruising speed" I shift mainly to stay within a quite narrow cadence range. As soon as knees or anything start complaining that's no longer a good cadence, so I shift down. If I want to go faster I start pedalling faster, then shift up as soon as it starts feeling inefficient.

I rode for many years with plain flat pedals and softsoled running shoes. I'd never go back, but at the time I felt I had a pretty efficient pedal stroke, pulling the pedal back as far as I could "grab" it. I like my spds, but mostly because I don't slip off them sidewise, not because I'm really good at using the upward stroke.