I did, at one time, buy Prometheus as an awesome villain. I'll tell you why: because he was a born and raised bad guy. He wasn't someone with a bad home life in the bad part of town who fell in with the wrong crowds, he wasn't some average joe whose life took a wrong turn, he wasn't a scientist who turned to crime because his grant ran out...he was a criminal from birth who really had no idea how to do anything else, or at least no inclination how to want to do anything else. That, more than anything, is what made me accept him as the "anti-Batman" character. He lived to be a villain the way Batman lives to be a hero, it was an all-encompassing lifestyle choice that existed beyond the death of his parents.
He's not a character you can take too seriously. I mean he's the kind of guy who would check "villain" for his ethnicity on a survey. You have to view the character as a Disney villain like Dr. Drakken or Dr. Nimnul. He's a guy who took down the JLA once because he made it his hobby, the same way other people put together the perfect deck of Yu-Gi-Oh cards.
But no one...not Sterling Gates and apparently not James Robinson...seems to get that. No one seems to write him as a bombastic forum troll who's ideas are nutty enough to take the JLA by surprise. They write him as this seething asswipe, and completely miss the point.
You say he's got no motivation beyond "be bad," and to me that's what makes him potentially fun. At his peak, Prometheus is a loser with no friends whose hobby is super-villainy; he's the archetypal pasty nerd, only instead of being fat and out of shape because he collects coins, Prometheus is ripped and healthy because his hobby includes martial arts and spandex. He could be fun, but he's not, and I doubt he ever will be.