Why do you say that? Bikes are pretty light, and the ones on mine seem sturdy enough. I was just about to get some sort of rack mount for my bike so I'm curious as to why you think its a bad idea.
I had a Chevy Blazer once with factory roof racks that would have been fine for a bike. This certainly isn't the case with my Lexus RX though. It just depends on the vehicle.
I wouldn't put my bike on the roof anyway but that is a different discussion.