I've never been a medical school interviewer so I don't know how they feel about thank-you notes. However, I think thank-you emails after someone takes time out of their day to meet with you absolutely makes sense. Just from personal interviewing experience, I've found that they can help in a few ways (not for med school):
1. If you have any key points that you thought were important but didn't fully get across during the interview, you can sneak that in
2. You can remind them about particular bonding moments during the conversation
3. It can tip the scale in your favor if the person is indecisive
4. It sets you up for future correspondences, should you need their help after getting an offer
I think that last point is the most important. Thank you notes are a way to communicate that if someone helps you, they will be appreciated. Communication of this point seems to me like a crucial element in mentorship and networking. I've had lots of mentors, contacts, etc. respond to a thank you note with extremely helpful information. For instance, I recently spoke to an attending about planning my first year of med school. After the conversation I sent him a thank you note. In response, he sent me the contact info of the chief resident, who had been a medical student here. Another senior resident responded to a thank you note with contact info of an attending whom I think I'll be doing my research project with.
In the particular context of the medical interview, after I got into my current institution, my interviewer responded to my thank-you email with lots of advice and offers to answer questions. Some of his words played a huge role in swaying me to attend the school.
I think it's true that nobody will care if you clearly do it just out of social necessity and not genuine appreciation. And that's fine. Sometimes you don't connect with someone, the relationship is coldly business-y, and that doesn't mean you won't get in. I wrote boring thank-yous to some interviewers that I just genuine didn't have anything to say to. But if you happen to connect with someone--and hopefully you will on the interview trail--then I think it's worth it to keep that connection open. You never know which small interactions will have an impact on your career/life later on. Should you find yourself in the position to email someone again 6 months later, it wouldn't be the worst thing if their inbox remembers you as a polite, professional student worthy of mentorship.