Here in GA, most people don't even seem to understand that the left lane on the highway is the fast lane. Every time I'm on I-75 for more than 5 miles I'll be going speed limit to speed limit +5 and get stuck behind someone doing speed limit - 10 in the left lane. I do not believe that the majority of people tapping their brakes are doing it for a valid reason. It's more often (based on them accelerating at the same time) because they're riding their brakes.
Mathematics has shown that each time a brake light flashes it forms a wave that miles behind causes total stoppage because for 2 minutes people hit their brakes lightly slowing down with each person behind slowing a little bit more.
I've been rear-ended by people following too closely who failed to recognize that I was slowing in time to slow themselves down. I'd rather be pragmatic and warn them off before an accident occurs.
By "will be slowing" I mean: I have removed my foot from the throttle or stopped cruise control but my vehicle has not started to appreciable slow yet.
Touching your brakes to flash your brake lights when braking with your engine isn't an altogether bad idea on congested road with traffic waves – it's a good signal to the car behind that you're slowing down (before they can actually see that) so that you can together help mitigate the propagation of the wave.