I don't presume to speak for Nick, but, FWIW, the rules of etiquette
on the open-source lists I participate in are pretty permissive WRT
the occasional public thank you (i.e., you rarely, if ever, see
someone reprimanded for this). In addition to the benefits you noted
above, the occasional thank you may also increase the likelihood that
people will respond to future questions (from any individual).
Personally, I'd gladly trade 100 thanks-yous for all of the postings
under incorrect subjects, top posting, vague questions, formatted
email, etc.
That said, I would never expect a thank you, nor take umbrage at the
lack of one.
In the killing 2 birds with one stone category, I'm often curious
whether or not advice actually worked (which is also part of the
FAQ). Closing the thread with the success/failure info also offers
the opportunity to give thanks. But like you, I don't recall ever
being outraged because I wasn't thanked.