I thought long and hard about that exact situation. Having been an EMT in Florida, the worst accidents I've seen were when people rolled their car and were ejected half way out of it, crushing and ripping them apart. I'd rather be kept in the car during a rollover than worry about the roof caving in. In that scenario, I'd be fucked anyways. There is no way I'm walking away from a rollover so devastating that the roof completely caves in.
And with a cage, you have all kinds of wonderful steel tubing to bounce your head off of during a rollover. I don't do cages without a helmet. But, to each his own...