Actually there is no need for ePub3.
However, why ePub3 appeared is a question that arrived from a plurality of reasons, only a few of them actually beneficial for the user.
1. ePub3 would need new hardware - good for manufacturers, not good for readers that need to buy (again) new devices.
2. ePub3 needs new software - good for software publishing houses, bad for authors, who need new software
3. ePub3 does actually converge to a multimedial concept, like a billion other formats, losing its special character (what is the difference from ePub3 and say a PowerPoint presentation???). Maybe it's not clear, but let's imagine a common AIO (multi-purpose apparatus that comprises a printer, a scanner and a fax): is this a printer with scanner and fax options, a photocopier with fax connectivity or a fax that can be connected to a computer and print files therefrom? This AIO came as a result of developments in all three initially not overlapping domains...
all these three clever schemes actually do nothing than to redirect an almost dried flow of money to their pockets. To make it sell-able, they invented a few things and claimed them to be the lapis philosopharum, one would be sincerely wondering why people could live without so much time
ePub2 has some shortcomings, but it was close to the intended purpose - to make a file look like a book - PDF was the first attempt, and it succeeded, but ePub solved the pagination on multiple devices.
I give an example - in the past, the entire satellite business was around a concept of CI/CAM: the end user bought what receiver he wanted, the he bought also the CAM he wanted/needed - this way the competition was free and worked in the favour of the end user.
By pretexting that the old CI could not cope with "age restriction" (which was a damn lie), the content providers issued a new format, misinformingly named CI+, note the 'plus', which introduced a lot of restrictions, and limited a lot the competition (the non-aligned Mascom went out of business as a result). The manufacturers appears to "happily adopted" CI+, as all new device was CI+.
I think I do not need to make the parallels between the ePub2/3 and CI/CI+ cases, at least 3-4 of them should be obvious.