Now, if you're looking for arguments whether or not you should implement them for a user-facing webapp based on a sampling of HN readers' behavior, my guess is we're not really all that representative. If you think any of your users might find them useful, go ahead and implement them.
One caveat, though: most (all?) random websites that want to give me notifications do so by just brazenly requesting permission from the browser. I would prefer if there was a link/button on the website itself that I could use to opt-in (which would trigger the permissions dialog). It just feels sketchy when random websites request notification access (or location access, for that matter), without first presenting me a case for why they want it.