As much as I'd like to mention my own/better solution, this isn't the venue for job boards to say "come to my site." We spam enough sites with our links as it is.
"Ask HN: Who is Hiring" is for actual job posts. Plain and simple.
It all depends on whether it's a job board that hackers will find useful and not something like Monster.com. If it's sufficiently relevant and useful to thread readers, it should be posted. Don't be pedantic, don't make up rules.
Pedantic might be a little harsh, but I see your point.
You have no idea how spammy job board people (yes, I'm part of this crowd) can get when they find a new field to peddle their wares. We're just shameless. And if there's isn't some throttle/downvoting, these threads will become 90% "check out my job board" and 10% real jobs.
You're making an assumption that all job boards are of equal worth to HN'ers, when in actuality there's a wide variety. I'm sure some job boards (for example Techcrunch's Crunchboard) would get upvoted, while other less relevant ones such as Monster, etc. would get downvoted.
To argue that HN'rs are incapable of distinguishing between job boards that they personally find useful from ones they don't seems a weak argument.
I'm happy to let the HN-ers looking for jobs decide by upvoting/downvoting the link as they see fit.
I've been posting my link in the monthly thread for maybe 4-5 months and it's been regularly upvoted, hence I'm assuming the users of the thread have found it useful. Especially as many of the people here are looking for startup jobs in the UK and relatively few UK startups tend to post in this thread.
Note how my post had three people upvote it before you posted your comment.
"Ask HN: Who is Hiring" is for actual job posts. Plain and simple.