to be fair, any phone you leave on with the light on for the entire time of use is going to have 3 hours or less. to some of us (me included), that's one of the big reasons to use an ebook reader instead. and since i get eyestrain when reading anything lit, that's another reason. the small screen seems like a good reason to use an actual ebook reader instead of a phone, too.
since everyone has different needs, wants and preferences, some people can love the iphone as an ebook reader while some of us can't even imagine using one to be anything but a pain.