This is an interesting subject. The two main issues that immediately come to my mind here are ones that have already been brought up.
First, you are right that the narration would affect the story. Hearing a voice is added information to the words, and I think can definitely affect one's interpretation of the material, depending on tone of voice, inflection, etc.
Second, there's just the simple issue of focus. It seems that when you read a book, there's a conscious effort to put aside time to sit down, open the book, and purely focus in thought on the material. Listening to an audiobook though, as you said, can be combined with other tasks, and I don't think this is as effective as reading just due to the fact that your level of attention is not as high. Not that it isn't possible...I suspect that if you were to close your eyes and meditate on an audiobook, your retention and understanding of the information could be as good, maybe better, than what you get from real reading. But one of the most basic things I've learned from psychology and my own experience is that attention level is incredibly important...many people may think they are paying attention, but there's that level of focus you can get to that really makes a difference in memory and understanding, which I don't think you can really achieve with 'reading' an audiobook, as long as it's accompanied by even menial tasks like jogging or something else.