Absolutely this! I think it does fall into a “you don’t know what you don’t know” until something happens to open your eyes. This happened to me as well, I will admit it, but it was many many years ago now.
Yes, I think the seesaw and heavy hands can be a shortcut to get a greenish horse and a novice rider into the training or first level show ring with a “frame.” Most of the riders (students and coaches) I watch over time don’t get past first level, and those that do move up the levels with scores in the mid 50s.
You can still bring home ribbons with a score of 56 at 3rd and impress your students.
People are indeed very touchy about any hints they are riding “wrong” and I have seen folks who water-ski on the reins claim their horses are light. So really you can’t do much but look the other way.
Take a look between 2:30 and 3:30…
https://www.youtube.com/watch?v=F_QlKdNrzAU
There are 3 riders who come in and out of the picture who are having obvious “conversations” with their horses, using the snaffle rein, with the curb fairly slack.
You can enjoy the rest of the video, but if someone wants to cut to the chase, that one minute…especially the rider coming in at 3:00 doing the 10m circle in trot is lovely.
Straightness is the first thing that comes to my mind. Yes you need your leg and seat but also the rein to dictate control of the shoulders and track. It can and should be subtle but it is still communicating.