Originally Posted by DaleDe
I would agree except in the case where the book requires characters that are not in the default map it may be needed, particularly for some languages.
I wouldn't say it's needed. For readers that allow setting the text font it's the same situation, because the user can always choose a font with the appropriate glyph coverage. It's only "needed" to work around a "feature" in some readers, that force a default font on you.