I totally disagree with that. The idea behind reflowable text is to allow the reader to set the formatting. Forcing a font on a reader is so Apple-like. Plus I guarantee a forced font won't have a uniform look or quality across devices as each device renders text uniquely.
I see nothing wrong with "forcing" a reader to see certain fonts for chapter headers and other special text (verse, song, signage, drop-caps) that may occur in a book... not to mention being able to use glyphs or characters that may not exist in standard, included fonts. After, all... what's the point of having the ability to embed (or choose) different fonts if it's considered bad form to do so. There still needs to be a level of artistic freedom in creating
an ebook... otherwise, what's the point of having the ability to style the text at all?
agree that forcing a reader to endure your particular font of choice for the main, basic text of the book is a bad idea (unless you need access to special characters that would otherwise be rendered as ugly question-marks or weird squares.)