Absolutely and completely not appropriate. Do not ever ask this of a med student host. The only exception I can think of offhand is if the host is a friend of the two of you and invites your spouse along just because they want to hang out with your spouse while you're in town. But to expect that of a complete stranger who is volunteering to host you overnight on their couch is not reasonable, as you obviously already concluded.
For interviews, it is more efficient for you to go on your own. There is no reason why your spouse needs to spend the time and money seeing all of the schools that you might not even get accepted to or be seriously considering. Most medical schools schedule second look weekends for accepted students in the spring, where they invite you to come back and look at the school again, meet students and faculty, search for potential housing, etc. That is when your spouse should accompany you to see the short list of schools and cities that you are seriously considering. Some schools even provide activities and information sessions for spouses of potential students; mine did.