Typically ebook readers are larger than PMP's with larger screens. If you're going to read for a few hours, then the screen of a typical PMP becomes pretty eye-straining (due to size, contrast, and brightness).
Amazon can do a lot of money only from selling kindle, like Apple does with the iPod.
The Kindle is already able to read more than just Amazon DRMed content, it can handle a number of formats.
The hack allow ENCRYPTED Mobi files to be read on the Kindle. So you can buy a DRM-protected mobi book and read it on Kindle.
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