Lets face it, people are lazy. Reading takes a lot more effort than listening or watching which makes video and audio much more attractive.
Mobility is another benefit of audio. You can listen to an audio book in your car or wherever.
The benefit of ebooks is mainly for reference. You can do a quick search to find the info you want to review. You can't do that with video or audio as far as I know.
Seems to me like packaging them together for sale makes the most sense.
What's your opinion?