At this point in the season, the ADCOM staff is tired. They have spent a lot of money, time and energy interviewing applicants. They have begged all of their interviewers just once too many to fill in for last minute cancelations, they may have had to deal with a lot of weather emergencies (even in a warm climate - bc interviewees have gotten stuck, etc!) They would not be wasting their resources if you had little chance of acceptance.
So I think that if they invited you, they are interested in possibly accepting you or putting you on WL in high enough position to be offered a spot. They know you are OOS and they are choosing to bring you in. They may need a few more great OOS candidates, or you may bring some diversity in terms of GPA/ MCAT score, home state, race/ethnicity or experience. Remember that some state schools need to get some higher stat OOS students to bring up their numbers. (and sometimes, they like the revenue the OOS students bring!) If a school usually has a very high IS preference, it is possible that they did not get a lot of great applicants from OOS, bc this is information that the smart students learn on SDN or elsewhere and many do not waste their money on these "donations". Unless you have an acceptance from a more preferable school, it is worth it to go on this interview!