Little Darwin

Although I have never used them, I am pretty sure that after a long ride, I'm not going to want to lift my bike to the top of my car... and being tired, I am far too likely to scratch up the roof of my car... especially if I remove the front tire, I would end up using my fork as a nice paint scraper.

Also, I have heard of people damaging cars and bikes by driving into a garage with a bicycle on top of their car, and don't recall ever hearing of bicycles being damaged by being rear ended with a hitch rack... You might do some damage if you collide with something while backing up, but you can see the bikes are there when you look back... you can't see them on top as you drive into a garage.

Parallel parking is a pain with a hitch rack... so I avoid places where I have to parallel park when carrying bikes.

I have heard that the gas mileage is decreased whether you use a roof rack or a hitch rack. Depending on car, etc... Roof racks have the bikes facing forward, so relatively aerodynamic. On a hitch rack, even though they are somewhat in the slipstream of the car, they are sideways, and potentially present significant frontal area to catch the wind.