I think on the one hand I agree with you guys about mockumentary style. I can’t say I’m a big fan of it. I think it can be used well when it’s a part of the story. When there is a reason for it.
I think that the Last Exorcism is a little different to films like Blair Witch and Paranormal Activity though. They are essentially “home video footage” compilations. The Last Exorcism, on the other hand, and I can’t say this for sure having only seen a trailer, looks like more like Man Bites Dog, where a “film crew” is setting out to make a doco about this last exorcism.
In that case, the style is very important. It also provides an opportunity to tell the story in a way that conventional filmmaking perhaps couldn’t.