Agree that this would be a good feature, and I’ve asked for the same. How I would use it - if I feel I’m going too hard, I’ll look at the HR display for confirmation. Otherwise, I don’t look at it. While RPE is very useful, sometimes it is also useful to get a second opinion from the watch. And should you notice that your pulse rate is high, or too low, that also tells you something about your physical state (e.g. maybe you are dehydrated).
So having the ability to display HR data overlayed on the map could be useful. I can see why some might want pace or something else (altitude?).