I think a lot of it comes down to the just-one-more-feature mentality. For example: MathML would be handy for technical books. I've seen mention of sheet music as well in these forums. I'm pretty sure the desire for multimedia elements comes from textbook publishers, who could use it as a selling point for their product (and it is sometimes a justifiable selling point).

Now if you're looking at ebooks and saying that they're just for novels, or certain types of non-fiction, then yes the current specification is probably fine. If you're looking at ebooks with the desire to expand their potential (even if it's just to the point of print books), then a lot of work has yet to be done.