HN users wouldn't notice the difference. I guess the main downside would be the owner of the submitted page might see a spike in traffic without knowing where the traffic is coming from.
Edit: Thinking more, maybe not a great idea across the board. It would make it harder for site owners to recover if HN is causing downtime (hug of death).
Sounds like a bad outcome - what is the upside to this ?
But it is a specific one-off action and should be called out
edit: that's nofollow not noref, derp.
All this is because there are a certain number of people who read HN who turn around and go out of their way to abuse the author (outside of HN).
noreferrer was a mistake and such has been said so elsewhere.
If Dang instead would block the domain, everyone would be happy.