There is an interesting article from the creator of the Topaz format where he shares some of the reasons that justify its existence:
http://beesbuzz.biz/blog/e/2010/01/0...evelopment.php
"The format has problems and it's arguably roundabout and circuitous for getting a worse result than a more obvious approach, but it has very good reasons to exist. If the books that are in Topaz format weren't in Topaz format, they wouldn't be in any legal ebook format. The point of Topaz was to prove to an old and established industry that ebooks are beneficial, and provide them an easy means to make it happen so that in the long term, everything will be available as a well-formatted ebook to begin with."