I have a real nice Jeep (Mopar) roof rack that is made from steel tubing. It holds my spare tire while i am towing my travel trailer, for obvious reason. It kills your mileage, even with the trailer disconnected the mileage drops a few MPG with just the rack up there. Anything you put on the top messes up the drag coefficient. The Liberty has a reasonably good aerodynamic drag coefficient; something like 4.0 I think it was. Jeep publishes it on their web site under specifications.
I too worry about the rear door hinges giving up due to the weight of the spare over the longterm, but in all fairness those are some pretty beefy hinges on our back door. I leave my tire there whenever possible. I am thinking about moving it to the back of the travel trailer so that i don't need the rack on top. I do use the rack for hunting trips, to tie my tree stands and so forth on top. The mileage suffers in my CRD when i do so too.
Silver Streak
`05 CRD Sport