Thanks, Jeff. I was one who indicated "must have." Here's my reasoning:
At a minimum, the use case in which the publisher's EPUB3 requires the preferred use of the embedded glossary should be addressed. That is, ereader developers need to know that their reading systems are likely to encounter this pedagogically-driven requirement. Publishers who assume that the glossaries or embedded dictionaries they build will be the default need to know that they have make it possible for reading systems to detect that.
Otherwise, to take your example, educational publishers are likely to create a host of proprietary ereaders/ebooks in order to enforce the pedagogical requirement, which in the US arises from the market (i.e., the state boards of education). If, with EPUB3, a publisher cannot meet the possible requirement to provide embedded age-specific glossaries with their Reading Programs, they will seek formats, readers or browsers that will enable them to do so.
Cheers,
BobB