Do you think an applicant would spend a day of their time (for current med students--a day out of their school's rotation), airfare in the hundrers of $$, to not rank you?
If you are going to interview someone and not rank them, I think some time of communication should be made. You should say..."sweetie we appreciate your interest in our program. thanks for doing an audiiton with us. but you are way out of our league/too introverted/too much baggage/you need to work on your ppl skills or something."
Honestly, I think unless someone is truly awful, I think they should at least get a rank.
People interview at a program, realize they don't like it and would rather not match than see themselves end up there, and don't rank that location. It happens. Obviously not very common, but definitely happens; I've heard from friends that are now interns about places they paid money (some to fly to) and were so turned off by the program/PD/scheduling/etc that they didn't rank it come February.
Also, remember that not everyone is flying to every single interview for residency. As a MS4, I've driven to > 50% of my advanced specialty interviews, and driven to all of my prelim interviews. Costs are real, but just because I interviewed at a place (that I would have absolutely no desire to end up at) doesn't mean I'd automatically rank the program (although in my case, I would rank every advanced position I got)
For example, if I interviewed at a prelim IM year that has 10 months of floor/ICU months, no elective time, and prelims are treated like crap compared to categoricals, I probably wouldn't rank it. I know that at least one prelim like this exists based on one of the advanced residency residents who did that atrocious prelim (not one I applied to, as I am too smart to apply to NYC prelims).
I think a program should not have to take an applicant that they objectively do not want (and would rather SOAP for a candidate if necessary), and I don't think an applicant should be forced to attend a program that they have no desire (and would rather be unmatched) to actually attend for residency. In both situations, it will lead to 3-7 years of tension from one side to the other.
Regarding your second paragraph, that's a slightly different issue. If someone does an audition rotation at a location and does well, they generally expect an interview. However, the reason for a mediocre student (who will not get an interview at that site even if they **** rainbows and act like a PGY-3, due to their scores/grades) to do an away rotation at a high-ranked place can be multifocal. For example, in the field I'm applying for, a LOR from a big name is a huge boon to an application, and doing an away in a specific region can show other less competitive residency programs (in that same region) that you're serious about possibly living in that area, opening up the possibilities of interviewing at other places. For example, I have a friend who did a Rad Onc rotation at MD Anderson in Texas (one of the top 3 Rad Onc programs in the country). He got a letter from the attending he worked with, and while he didn't get an interview at MD Anderson, he interviewed at a multitude of other Texas hospitals, and actually ended up matching at a Texas hospital come March. All of this was as a student from the northeast who had no personal ties to Texas.
However, what you're suggesting is in practice on a small scale: Penn Derm, I believe, has put a disclaimer on their visiting medical student page stating that doing an audition rotation at Penn Derm will NOT guarantee you an interview at the program. I feel like this is generally understood and is somewhat common sense, but I'm sure they've had issues in the past, so they addressed it directly on their website.
All of this talk, however, is kind of moot. Most programs (I imagine, maybe some residents can chime in on this) will rank you if they interview you. That being said, I'm not sure what the practical difference between being dead-last on the rank list and not being ranked at all is (especially since the programs that would likely not rank you are the more competitive/coveted programs within their fields).