On occassion during rides I've begun tapping into my stoker's Garmin Ant HR belt ID (we each have an Edge 800) just to see what she's actually doing back there. Sometimes her HR is way too high, sometimes low. Her perceived effort rarely matches her HR level %, and it is difficult for me to tell by feel how hard she is working. When we set out with a goal (easy, mid, hard), this seems to help herd the cats. Other feedback such as excessive panting or talk are two additional inputs I use to gauge her
I've speculated, knowing a competitor's ANT ID could be quite interesting in a race. You'd see when they were pegged or not. Our belts seem to work from ~80ft away, so there is a lot of stealthness available
