A thank you is far from mandatory, but not writing one is a missed opportunity to be seen as an excellent candidate. The thank you note isn't about gratitude (though it's included), but it's about an additional opportunity to reinforce the impression you made or put out any fires you started.

Smart candidates should see the thank you as a self-marketing opportunity.

It's probably not required if you're seeking just-another-job from just-another-hiring-manager.

However it's probably a good idea if you're interviewing a founder (especially of a client-based business) who's seeking people who share at least some non-zero fraction of their passion for the business.

I wouldn't suggest faking enthusiasm. But if you genuinely feel like it would be one of the best places for you to work, and you made that clear during the interview, it doesn't hurt to follow-up and say it again.

EDIT: The original "who the f--- writes thank you notes?!" comment was deleted while I typed this.

Thanks for the insight nonetheless.

It was common advice that "everyone" gave when I was in college. I've never liked it as interviewer, so I've never done it as an interview-ee.

What is wrong with making an effort to get a job you want?

Considering the situation, doesn't it always seem more like trying to suck up than genuine?

Sounds like an issue of pride.

I personally find it slightly awkward/annoying. Wouldn't hold it against someone, but would prefer they didn't send it.

Despite people's advice to the contrary I tend to agree. I doubt I will ever send a thank you note unless it is somehow warranted.

No.