I think there is always some confusion about this issue. From what I have heard, people often consider what you call "the interview" two separate events in the interview process.
Your spouse/significant other is going to be greatly affected by where you choose to go to medical school (if you have a choice, lucky you!) You should definitely bring them out to the "interview" that is, the entire process over a few days. If you can spend the extra time, have them come with you on a tour of the city, visit some local recreation areas (you may not have much free time, but need to spend some time with them or suffer the consequences). See what the neighborhood around the school is like. Are children a consideration, either now or in the near future? If so, what schools/day care, etc is available nearby. Is there a good job market for your spouse/SO? Do you both like the climate and "feel" of a city? These are all important issues that you both should consider. This may limit your accomodations during the interview (no staying with students I would guess) but you should do your best to bring them along.
However, they should NOT come along with you during the actual "interview" the time you spend at the medical school, on the official tour, mingling with other applicants, and the interview with faculty itself. I've seen it some myself, and heard much more: applicants who bring parents or boyfriends/girlfriends along with them during the whole day they are interviewed. Parents seem very overbearing, and make the applicant look immature. SO's are usually bored out of their skulls (they won't be interested in how many students there are to a cadaver) but don't reflect as poorly on the applicant.
So, what is the message? Bring your spouse with you for the whole process, but leave him/her someplace else while you are getting interviewed. Feel free to give them an informal tour, or explore the campus yourselves, but don't bring them along on any official functions provided by the school.
