Well, like I may have said before, they might not only be doing this just for big bucks and profit, but as I recall, it's been probably about some 10 years since RotK graced the big screen...and now, with this new movie, this prequel, PJ and company are probably thinking: "The fans have waited ten years for only one movie? That would seem like an injustice to them. Let's give them a substantial reward for their patience."
My roommate, who is a big time fan of the LotR books, and all others associated with them (The Hobbit, The Cimarillion....sp?) seems to think that one way Jackson could stretch out The Hobbit into a fairly decent trilogy is to utilize elements of The Cimarillion as backstory woven into scenes in The Hobbit. The Cimarillion, as I understand it, is not so much a novel to be read, but a source book of history and back story for Middle Earth.