[dcminter]

If you were a low-effort spammer (Edit: I'm not suggesting you are a spammer btw) you'd never notice and keep on posting spam into the void. Whereas if you were notified you might create new accounts ad infinitum increasing the administrative burden of HN.

I once implemented a similar feature for the comments on my blog along with an embarassingly simple filter to identify the spammers initially. The volume of spam on a no-name nerd's blog and the degree to which this approach resolved it were both surprising to me.

On a later incarnation of the blog I just removed the commenting feature, but I guess that wouldn't be a sensible strategy for HN :)