Peope seemed to have neglected ol' Clive Barker's Pinhead.

Just like Jason and Freddy, you keep killing him, and he keeps on coming back.

And on the book thing: KBFX, the effects team from Evil Dead 2 team, did the effects for Jason Goes To Hell. I haven't checked, but but they possibly did the effects for Creepshow, too. Maybe they couldn't be bothered to create extra props, so they dusted off the old stuff.

Another posibilty: Jason was trying to open the gateways of hell, but after the amount of beatings he took from the previous films, he can't speak, but being undead, he can read sumarian. He needs someone equalkly as evil to communicate with and read the incantations - .maybe not every exact syllable - so enter Freddy.

And who else can stop them, but Ash?
What kind of world are we living in, where a man dressed as a bat gets my pulicity?