I think the emailed thank-you has taken off in the last 7 or so years. When I was in a position to do hiring 10 years ago, I maybe got one handwritten thank-you per position I was interviewing for. The thank-yous usually came from people who were not in the running for the job after their interviews, and did nothing to sway the opinions of the interviewing committee.
But more and more, I'm seeing the thank-you as an expectation, and as can be seen from CharlieBraun's post, a necessity for some companies. IMO, rejecting an otherwise qualified candidate because they didn't email a thank-you is not a good idea, but this isn't the first time I've heard of it happening. So I think there will be more and more job searchers sending thank-you emails, because there is no way of knowing which companies demand them and which don't.