Not everyone using an iPad will want to use iBooks and thus can fail on a number of different apps that handle ePub. The Kindle app will also fail on an iPad.
All eInk readers that use ADE to display ePub will fail. That means Kobo, nook, Sony, Onyx, and other brands. It also means a number of apps to display ePub on Android will fail.
For now, using ePub 3 is a major fail because support is very very limited.
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