Actually, in the Spawn comics, there is a medieval version of him. It isn't the same Spawn as the modern day one, but a fallen knight who was given the Spawn persona.
But yeah, guest characters are sucky in that they rarely have a legit reason for being in the game which I feel is ruining the experience. Though, Namco DID try and make some storyline explanations for all guest characters, E.g. Heihachi found a sliver of Soul Edge which took him back in time, Yoda and Darth Vader have travelled through the galaxy to get to Earth, etc etc... Sucky storylines, but at least they tried...