2 years ago we matched 1 student who came back and spent 1 or 2 days after the interview for a 2nd look and last year we matched a student who called and spent 1 week with us prior to the interview.
It definitely shows interest in the program and although you cannot know these students like you do the full month rotators - it helps to differentiate them somewhat from those who you only meet on interview day.
On the other hand, I do not want to leave the impression that it is in any way mandatory nor does it guarantee moving up the rank list. Final analysis, if you are interested in a program but cannot spend a full month there it is worthwhile to try and spend some additional time as it allows you exposure to the residents and some of the faculty.
As you know from this site, however, there are programs which place a very high premium on rotating and you need to factor that in as well as you make your choices on where to do an AI and if possible where to do "one week wonder" visits prior to the interviews or 2nd look visits after the interviews.
Good luck -
wnl