The essential point is that while fixed layout gives the author and designer more control of how the finished e-book will look, fixed layout is not supported consistently across e-book readers, and is therefore not accepted by the great majority of e-book sellers who require reflowable text.
From Wikipedia (August 2015)
Comparison of e-book formats
The EPUB format has gained some popularity as a vendor-independent XML-based e-book format. The format can be read by the Kobo eReader, BlackBerry devices, Apple’s iBooks app running on iOSdevices, Google Books app running on Android and iOS devices, Barnes & Noble Nook, Amazon Kindle Fire, Sony Reader, BeBook, Bookeen Cybook Gen3 (with firmware v2 and up), COOL-ER, Adobe Digital Editions, Lexcycle Stanza, BookGlutton, AZARDI, FBReader, Aldiko, CoolReader, Mantano Reader, Moon+ Reader, the Mozilla Firefox add-on EPUBReader, Okular and other reading apps.
Book Services can provide both fixed and reflowable layouts of your book. An EPUB file using fixed layout is simpler to generate from existing printed book layout than a reflowable EPUB file.
The simplest way to distribute an e-book is to use the Ingram Partner Network of 70+ e-book retail & library partners. This requires a valid reflowable EPUB file and cover file, created within outlined specifications that satisfies the requirements of each partner, getting your e-book where it belongs—on every device.
PROFESSIONAL E-BOOK DESIGN AND LAYOUT SERVICES ARE AVAILABLE AT AN HOURLY RATE OR AS A PACKAGE. Discounts are available when combined with similar print book packages.