While I disagree that OCI interviewers have already made up their mind about which interviewees they will recommend, I do agree that thank you letters are unnecessary. I work at a large firm and have interviewed a fair number of candidates for our summer program, although all of my interviews have been at the firm or over lunch, rather than at OCI. I don't care about thank you letters and my impression is that no one else does either. I don't recommend sending thank you letters because I don't think they will help you, but they may hurt you if you make a mistake in the letter.
Do you disagree because you have some knowledge or experience that leads you to believe otherwise, or because you just don't want it to be true?
Why would I not want it to be true? I have a job and I will never have to go through OCI again. Many of my co-workers do OCI interviews and we have discussed this. At some schools my firm will pre-screen the potential applicants, but the OCI interviewer has complete discretion to recommend whomever he or she likes, and management virtually always follows these recommendations.
In my experience, and other people I've talked with, have very strongly felt that some firms DO approach OCI with a shortlist of people who are already getting offers for callbacks, and the rest will be rejected absent some extraordinary interviewing skills. I've felt like this in both interviews where I did get callbacks (and offers) and some in which I got a rejection dated the very same day as my OCI screening interview.
So some people who are perhaps unlikely candidates to get a callback (maybe at a school where the students get to choose firms via lottery or something?) might hope that the pre-interview decisions don't in fact take place. So I was wondering if you were merely disagreeing because maybe you were one of those applicants, or if you had talked to people who interview and had information to the contrary, which turned out to be the case. It wasn't in any way suggesting that you didn't have a job or something like that (or if you read it like that, I didn't mean it that way), but instead just looking for some more information on your position.
Yeah, I can understand that the process seems a little opaque. To be honest, after interviewing candidates and discussing the process with my co-workers, the process now strikes me as somewhat arbitrary. For what it's worth, I generally just try to have a conversation with applicants -- about the firm to an extent, but also about current events or whatever else either of us wants to discuss. I also always conclude the interview by giving them a frank assessment of what I perceive to be the pros and cons of my firm. I do this to give them the information they need to compare my firm to the other firms they're interviewing with. I do this when we're wrapping up so they don't feel a need to respond, as I can understand how that might be awkward.
I also should admit that I don't know how other firms approach recruiting. Some other firms may in fact decide in advance that, absent a stellar performance at the interview, certain students won't be receiving call backs. It would surprise me if this was common, though.