Even if lots of the content isn't work related, plenty of it definitely is, or at least it can help you to look at something that might jog your head in to gear again when you're stuck.
And countless times I've used 'Ask HN' style posts to help me to overcome some kind of problem or to ask for a recommendation about what tool to use.
It's really up to the user, which content you choose, and scanning the headlines and only picking out those items relevant for the work you're doing is something that is just a matter of discipline.
If you're being paid to perform duties or hit milestones, then that's more important than whether you are focussed only on your work when you're at work. A needed break or relevant distraction like HN shouldn't be a problem.
I use the Alt-Tab rule when making this decision myself. If one of my team mates walked past, would I Alt-Tab away from this screen or would I be fine to let them see what I was doing?