Backward compatibality will always be a problem in the digital world. I guess if there will be books with audios and videos, there will be calibre version 15 with ability to convert to any format that we want..
I can definitely see the utility of these books as anamardoll has already elaborated. It would be interesting to see if the ereaders will progress the same way. Not that I am unhappy with the current status of ereaders but its fun to think..