One advantage of a "pure" hybrid is that the little fuel-burning generator engine can be designed to run optimally at a steady speed. You'd be amazed at how much of the crap on a normal gasoline car engine is designed to make it run optimally at different speeds and load conditions.

I see no reason the "generator" engine couldn't be designed in a modular fashion so you could request one that burns your favorite fuel. Maybe some versions could burn several different fuels, but natural gas requires a pressurized tank, so it's a bit more complicated than just dumping either gasoline or ethanol in a normal fuel tank.