Sorry, my comment was rather lazy. Let me try again.
I get the impression many of the people who want to start a HN like clone are considering using it as a content delivery platform. For example http://arclanguage.org/item?id=20452 notes a revenue sharing model for specialized content. Now, if that's the case these site owners will have both spammers and content scrapers to contend with. So my initial comment was also referring content scrapers too.
However there's another thing to consider: Session storage costs. About 6 months ago I went through a process to reduce the cost of data held in memory for session storage (redis in this case). The session data was continually analyzed for determining both who the bad users were and for knowing what value the good users are getting out of the app (feature planning etc). It was an interesting process, where just by reshaping the session data, thousands of dollars per month could be saved in db fees. Now I realize the someone starting a HN clone is probably not dealing with that, but I'd be willing to bet that part of the reason Captcha was implemented in HN proper was to reduce fees associated with the volume of requests, session costs and even network load. It's my feeling that, generically speaking, adding captcha functionality is a good Option to have.
> I'd be willing to bet that part of the reason Captcha was implemented in HN proper was to reduce fees associated with the volume of requests, session costs and even network load.
Let me add some info to that.... I'm not sure if anyone noticed, but HN has implemented new session management strategies. You can see this as your login is now maintained across multiple devices, where the arc code (that we have access to) logs you out upon logging in elsewhere. I also believe that when pg handed over the HN code significant changes occurred including how session data is stored and how that data is utilized to integrate with cloudflare. Obviously I'm making big guesses, because I don't have access to the code, but I'm willing to bet the changes HN has put in place would surprise everyone here.
Sadly everyone who sees HN today will come here and look for the source code not realizing what's available here is not modern nor comparable.