Aside from the "I'm a Cheap Bastard" argument, what is the advantage? They look "Cool?"
Better handling? I seriously doubt it. A friend of mine had a chance to ride a Rocket 3 with a car tire. He said it cornered like ****. This was in town. There's no way I would take it anywhere handling might actually matter.
Better ride? If it won't turn, why would I care if it's cushier. T-birds aren't exactly harsh.
Does anyone honestly think they will work better than a properly designed motorcycle tire? It's all just an attempt to justify being a cheapskate.