InDesign is a semi-professional print publishing application. Like most of Adobe's major applications, InDesign is expensive because it does so many things, and because the market can bear the heavy cost (most companies write it off on their taxes). ePub generation was added to it much later, when the product had already been on the market for years.
There is no real value to buying InDesign simply for ePub generation... it is a waste of money. Other apps can do the job as easy or easier, and many of them (as you pointed out) are cheaper, or free. In fact, if you know HTML, you can roll your own and zip it.
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