I actually went on school visits with my mom. The AdCom at Cambridge said that he was pretty sure that was a first
I personally appreciated having a second opinion to bounce off ideas. She didn't join me on any of the tours, info sessions, etc., but she did attend the dinner the evening prior (at Cambridge). We didn't interact much at the dinner - she chatted with the people on her side of the table, I chatted with the people on my side of the table. But then we could talk about it afterward and compare our thoughts. It was also nice to have someone to travel with. Granted, I see my mom once a year, so I was also taking the opportunity to hang out with her on this trip.
Bottom line - if you would appreciate a second opinion or if your significant other is interested in seeing the schools, bring her along. She may also see a different side of the school that you won't get to see. But I agree that she shouldn't attend the actual info sessions, tours, interviews, etc. Boring for her, distracting for you.