The issue with local search is that you anyways end up doing search on the server when on mobile or when you are not on your home computer. But yes, definitely useful. I personally do 0 inbox so I hardly have anything to search, heh.

Regarding the submit to HN button: if the article has already been posted, HN will itself just link you to it rather than make a new submission. So it is a bit of an intelligent hack where it piggybacks on HN's own code, hope that assuages you :)