Have you read Mira Grant's "Newsflesh" series? For those not in the know, "Mira" is actually UF author Seanan McGuire, whose Toby Daye series is a peer of TDF, and the lady knows snark, epidemiology, the whole damn horror genre and how to spin a cliché back into something interesting. "Newsflesh" is easily one of my favorite series of the past decade.

I'm OCD enough that going the other direction, saying that one mythos obtains while others don't, bugs me; I can't see why just one could be true and no others. Plus, it takes a big pair (of cerebral lobes, duh) to take on all the world's mythology and at least make a nice dent in incorporating them. At the very least, it makes a wide-open, potentially HUGELY rich resource for the author when he doesn't just limit himself to one take on one mythos.