I think open houses are very important in getting to know the feel of the school. I have only been able to go to the ones for the 2 med schools in my state, but I felt much more comfortable on interview days with then b/c I had already visited. Plus its nice to see what "kind" of students are common at the school. Every med school has a different vibe and feel. Its important to find one thats a good match for you.
Plus it allows the office staff to become familiar with your name. I dont know if it really helps in the process that much but its nice to know they recognize your name/face when you show up for interview day as opposed to never having met them in person before.