Training. That brings up another pet peeve I have.
Heart rate monitors have revolutionized training for endurance sports. The fact that they are inexpensive and widely available has also taught us some pretty wild things about physiology as well. Which is always kind of cool.
The fact is that with proper use of a heart rate monitor, you can train much more efficiently and your training can be more personally focused rather than a one-size-fits-all training program. That won't matter just for super athletes, but also normal people. It could mean if you were exercising five hours per week, you could get the same (or often better) health benefits with three hours per week of more precise effort. And that wouldn't necessarily be more intense effort either.
All of that assumes you can use an HRM properly, and have the proper information and tools to make use of it while you exercise. Unfortunately nobody makes the app that puts all of that information together in a format that you can use while you are riding your bike (or running, or swimming, or cross country skiing). And some of the numbers you need to figure out (like your maximum heart rate and VO2 max) are hard to estimate reliably, and measuring them basically involves torturing yourself. And for the whole thing to be effective you need good estimates of those numbers.
It is just kind of a bummer that nobody is making that app.