The harder you're breathing, the more oxygen and food energy you're consuming, and the more Carbon Dioxide(CO2) is building up in your blood. That CO2 then has to be exhaled and Oxygen has to be inhaled. This is a continuous cycle and your heart rate is only a proxy for it.
Even if it were useful, you can already feel your heart beat if you are concentrating on it. Even if you lacked this ability you can feel, at least, changes in your heart rate. Even if you couldn't do either of those things you could just wear a heart rate monitor, they are cheap and readily available and have no side effects.
If you're using those molecules to think or move, wouldn't it be nice to have a sort of tachometer?