Just a couple of thoughts:
Whether you believe ouija boards (or other devices) are legitimate tools for communicating with other entities, or if you find them to be silly toys people use to fool themselves, it seems very hard to disconnect the user from the experience. For people who claim to have dramatically negative events in their life after using a board, I'm not saying it didn't happen, but I would really like to know what was going on in their lives anyway, before the ouija board. As Guiley mentioned in the interview, there are quite a few unstable personalities who are drawn to paranormal studies, and I wonder if their experiences don't skew the collection of stories in a certain direction. Moreover, human memories tend to merge with one another, e.g., a group of people share an experience. Interview them immediately and separately, and their accounts differ. Let them talk amongst themselves, and in time their accounts tend to become more unified. Elderly combat veterans sometimes even have memories of anachronistic elements in their service history, often influenced by film and television.
Again, I'm not discounting anyone's experience, I'm just arguing for the role of expectation and malleable memory. I once attended a presentation by a well-known ghost researcher who, before playing his collection of EVPs, handed out a script of what the EVPs said. To me, the EVPs sounded like "mkjhniugcabcn," or static, but maybe if I put more faith in the script I would have heard, "I am a carpenter, and this is my brother John."
Another thing I was thinking of is how people sometimes say that something is just a product of the imagination or the unconscious. The part of this I would disagree with is the word "just." In my opinion, the imagination and the unconscious act as quite potent forces within people, not just with ouija boards but in life in general. As someone suggested earlier in this thread, one may sometimes externalize things from within yourself, and it's easier to accept that occasional malevolence as an Other.
I watched a bit of a documentary about a horse trainer, maybe the horse whisperer dude. He said something like, "A horse is a window into the owner's soul." That may sound corny, but did you ever notice that people with problems tend to have problem pets?