My thought is that they shouldn't be rotating students who they wouldn't interview. That's half of the reason you rotate somewhere - because it's one of your top choices and you want to see if you really like it. But if you aren't going to get an interview . . . what does it matter whether you really like it or not? Another reason you're rotating is because you want to gain an extra edge in the selection process. Yeah, you also do a visiting rotation because you want to see how EM is practiced outside your institution and because you want a great education, but you can get this at 100 other places. You only have 1-3 months for these rotations, so I would want them to be at places I actually had a chance at. Just my thoughts.
I hear what you're saying, but I agree with socmob that an away rotation is as much an opportunity for you to evaluate the program as it is for the program to evaluate you. An away rotation is defintiely a risk, but a risk worth taking in most cases.
To my knowledge, there is no prescreening of applicants to do an away rotation here and no required board scores, letters of recomendations, applications, etc. like some programs require. Even if those things are up to par, some med students just don't work out when here in person. Some just aren't very good, some are good students but have bad attitudes or other personality problems, and some just don't work out for a variety of other reasons. There are tons of reasons why a program might like a student or not like them, but those are very individualized to the situation.
Lets take an extreme example of a rotating student who does a poor job and has a bad attitude throughout the month. Does that student automatically deserve an interview? Are they entitled to that simply because they rotated there? As someone who is applying who didn't rotate here, would you like that person to have a spot (and potentially your spot) when we all know its not going to work out for them? As an interviewee, would you rather have a token interview or would you rather have an honest answer up front before you waste your time, money, and effort to come back for an interview? Wouldn't you want that time to interview at another place or spend that time doing something more beneficial? As a program, if you have a signficant number of similar students, do you want to give away a large percentage of your interview slots to students who are very unlikely to match there after their performance during their month? My answer to all of these is "No," but that's just my opinion. This is an extreme example of course and most rotating students who don't get interviews don't fit this profile at all, but I'm using an extreme to make a point.
In the end, I guess I value honesty above courtesy in a situation where there are limited interview spots and limited time/funds for interviewees throughout the year. Again, just my opinion.
I think it's logical to ask if programs should screen away students before they start the rotation. I guess I look at medical student rotations as more about education and teaching from the point of view of a program, as opposed to a primary opportunity to find future residents (althought both are important). That's definitely a debateable point and I'm not sure there's a "right" answer for all here.