Something I find very useful about reading a site like HN is the little seeds it plants in my mind: I read about some interesting program/ language/ library/ framework/ technique/ theory or whatever, and then, six months later, I find myself needing such a thing and remember, isn't there that thing I read about?
Also, I'll discover something I maybe didn't even know existed and realise that I should learn about it; and so take a mental note to do so. Then, later, when I have the time and inclination to learn about something new, it's there on my list.
I agree. Every so often there's a post about something that can change my approach or direction to a problem I'm solving. Even if it's not relevant to what I'm doing, there's a wealth of information that's worth knowing. In a nutshell: HN is an extraordinarily good place for a tech entrepreneur to find out what he doesn't know.
However, in practice, I find it still takes a substantial amount of effort to filter the information I'm exposed to into something I can use in the near future. The amount of due diligence in figuring out if a certain technology (especially if it's new and if you lack experience with it) is a large enough barrier for me to usually continue on with what I'm doing. I simply don't have time to learn a new language every month, or learn how to migrate to new DB, or any infrastructural changes like that.[1] On the scale of things, I've prioritized "get stuff done" as #1, with "learn how to get better stuff done faster" as #2. HN is good at #2.
Like anything, HN is what you make of it. I personally love the fact that HN is so densely populated with the exactly the type of people whose opinions I actually respect (surprisingly rare for me), and who I'd like to network with. As a result of the amazing crowd here, I get to learn what I don't know at an astounding rate. From there I can leave it to my natural curiosity and ambition to fill it in.
Edit: [1] Another thing is, quite often people post things on HN merely seeking validation of what they're doing. So many blogs I read are about "Here's how I do something, and why you should, too. (And since I'm telling you to do it, it means I'm right)" which puts an even huger burden of due diligence on my shoulder. So I might have heard of xyz... but still don't know if it is worthwhile.
Also, I'll discover something I maybe didn't even know existed and realise that I should learn about it; and so take a mental note to do so. Then, later, when I have the time and inclination to learn about something new, it's there on my list.