As a reader, I don't want multimedia and even if I could read the eBook without activating it, it would still be there as code bloat unless I went and stripped it out. This means that I would have to strip out the multimedia from most ePub 3 eBooks. That's just something that will bring about a change back to pBooks.
I think you may be confusing the ePub 3 standard
with an actual ePub with the multimedia included. An ePub 3 compliant reader will display all ePub2 books and support
the multimedia/mathxml etc of ePub 3 - it doesn't require it - so there won't be any need to go back to pBooks. If you are worried about those extra "features" that the author intended taking up too much space on your device then you always have the option of NOT purchasing the book...or stripping it out if you absolutely want to...but that doesn't mean that there aren't people who have a perfectly legitimate need/desire for the functionality allowed with the new standard - as the OP stated he has run across several.
While I agree I don't really care to have someone else's sound track playing in the background while I read a book - a young reader might enjoy "mood" sounds in a mystery. I wouldn't want to take away that readers option to have it. I can also see a great reason for embedded video if you are making a "how-to" book with examples. I also would like more of my eText math books to display the equations properly instead of being stuck with a PDF format. Sooo - having the CAPABILITY to do these things in ePub3 is a good thing but not mandatory since it will display ePub3 compliant books without them just fine.
_Savage, I hope you let us know what you find out!