If it wasn't for Amazon adopting this format for the Kindle, we wouldn't bother creating this file format. It is used on some other minor ereaders that have been used in the past, but when a modified version of it was adopted to use on the Kindle back in 2007, it suddenly became an important ebook format to have in one's list. Not necessarily because the "Kindle Direct Publishing" requires this format to upload a book into their service, but because if you sell directly, people can "side-load" this file onto their Kindle and read it. Between the EPUB and the MOBI formats, you are covering the main two ereaders people use most, and the bulk of non-dedicated ereader devices.
As of this writing, Amazon has recently announced their new line of Kindles, including the Kindle Fire, which is planned to be updated to use HTML5 instead of the MOBI format. It will be able to handle graphics for children's books and the like better than the MOBI format. As that develops and gets implemented, I plan on updating this book to include creating ebooks in that format. However, according to Amazon, the updated ereaders will be backwards compatible. Which means the files you create now will still be readable on the newer devices, and they will still be able to read files loaded on them in the MOBI format. For text-only books, there will be little loss to worry about updating then into the HTML5 format when it comes out.