It's not a nice position to be stuck in, but there are plenty of BJJ positions that are awful to be stuck in too. Ever been crossfaced? It's not nice. The key difference is the timing of the tap.
If someone catches a heel hook, before you actually feel enough pain to warrant a tap, it's too late. You're either okay, or your knee's completely blown out. Which is why so many competitions make it illegal and many gyms won't teach lower belts heel hooks. Should they? In my opinion,
yes. The sooner you can make your students aware of it, they can learn the dangers of it, learn the escapes, counters and how to ultimately avoid losing their knee.
Many academies teach a 'catch and release' system with heel hooks. You both know you don't need to crank it on, if you've got it, you've got it. Even if they don't tap, just let go.
Yes accidents happen, but they do with all submissions, I've had my arm broken and been choked unconscious, simple mistakes usually led by (my) ego.
That's where we got our name from, to keep it illegal would actually make us continually seem cooler, but, we want to set the Reap free.