When I was younger, I used a lot of pre-packaged adventure modules with the RPGs I played with my friends. One of the things I appreciated about them is that, if they introduced a new monster or magic item or whatever, they included the game stats for it at the back of the product. Come to think of it, that was generally how new "stuff" got introduced into RPGs. There were exceptions, of course. For example, Gary Gygax's module, The Forgotten Temple of Tharizdun, uses a lot of monsters from the Fiend Folio, so, if you want to play that module to its fullest, you need a copy of the FF. Likewise, Isle of the Ape uses spells and magic items from Unearthed Arcana and more or less expects that the referee owns and is using that book in his campaign if he wishes to run the module.
How do you feel about this? At what point is it too onerous to expect that a referee owns a particular supplementary work? Should adventure modules include everything not included in the main rulebook for the benefit of those who don't own anything else or is it reasonable to assume that most players own the latest releases?