Whenever we print rules-related material in Dungeon, be it a monster, a feat, a spell, a magic item, or whatever, from a non-core (basically a non-SRD) book, we reprint all the rules you need to use it as it pertains to the adventure in which it appears. That's not going to change. It can't change. The magazine has to appeal to the widest base of readers possible, and every new "must have" book we add to the core list reduces that base.
For something like monsters, it's an easy issue since stat blocks are pretty self-contained. We already have to print full stat blocks for classed creatures and advanced creatures, so printing full stat blocks for creatures outside of the Monster Manual isn't a big deal. And in the end, monsters are the most important rules bit that appears in an adventure, so it makes sense to pull monsters from as wide a range of sources as possible.
It gets less easy with feats and magic items, but generally, NPCs have a relatively few amount of these things, so it's usually bearable to reprint one or two here and there as they pop up.
Spells, on the other hand... well, when a monster has spells, it has a LOT of them. And unlike feats (which we can summarize in a 2 sentence sidebar) or magic items (which take more room), spells can't really be easily summarized without taking up an unforgivable footprint in the magazine.
That said, we do include new spells (or pick up spells from non-core books) in the magazine from time to time. It's just not common. For adventures set in the Forgotten Realms or Eberron, we add the appropriate campaign setting hardcover to the "core list" for that adventure, which opens up all the spells in those books for that particular adventure. When we run an adventure that heavily uses a new WotC book as a resource, we do the same ("Hellfire Mountain," if I recall correctly, has a few reprinted spells in it.)
There's a final problem as well, and that's the simple fact that we editors at Paizo honestly aren't familiar with every WotC book. We don't have the time to be. Given the breakneck pace of magazine publication, there's just no way for us to read every book that comes out and then decide which new rules elements could spruce up an adventure. And limiting the adventures, by and large, to the three core books (with occasional stints afield) allows us to do what we do without worrying that some spell in some new book completely undoes the entire plot of an adventure.
Personally, I'd love to make the Spell Compendium, Monster Manual II, Fiend Folio, and the Complete line of books all "core." That'd free up a HUGE amount of room in our adventures to let them be just that: adventures, rather than adventures pocked with patches of reprinted material. But again, we just can't assume that every one of our readers has access to these books. The hobby's expensive enough as it is.
Hope this answers some questions. If there's a point that I forgot or neglected to address, let me know in this thread and I'll reply.