I'm not sure if I get the MathML point but if it allows for better mathematical formulas display, why not. As long as we are talking "books".
I can see the point of epubs supporting sounds: if I use my reader to learn a new language, it would be cool to be able to tap on the example and have it read to me. But that's belongs to the "nice and shiny new toy" features, not the required features. So if it's dropped, well, too bad, I'll just have to keep using my mp3 player to read the file... Especially if my reader has no mp3 player ^^
You probably could find an example where it would be cool to have an embedded video play on your reader, but the hardware isn't there yet. Unless epub3 is intended for tablets and computers...
I'm all for updates that allow for a better book display, the rest should be optional...
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