I posted this very same question recently on roadbikereview.com forums.
DH and I carry our bikes inside our Escape and wanted to move them outside. We have kayak carriers on the roof and are hesitant to keep swapping rack accessories back and forth for our various toys. We were debating roof vs. hitch mounted carries and opted to get a Thule hitch-mounted carrrier (the T2, I think it's called - holds the bikes upright, attached at the wheels, not hung from the top tube) for our 1.25" receiver hitch.
Some benes for the hitch:
It's lower and easier to mount vs. roof racks with tall vehicles.
It allows you to keep the roof clear for (a) other sport accessories, or (b) air flow and better gas mileage.
They are - or can be - super easy and fast to load and go.
Frees up the interior of your vehicle for passengers (imagine that!) or luggage.
Some cons with the hitch:
It uses your bikes as a bumper. If you get rear-ended, your bikes (and maybe your vehicle) are toast. But....a rear end hit is usually someone else's fault, so their insurance would pay to replace your rides. A small consolation when you are ride-less as you settle up with insurance.
Although they can lock, I'm leery of the safety of the bikes on there. Would they be easier to steal? I don't know. You could always back the vehicle up to a wall or something if you need to park for an extended period of time. That would be true for any rear-mounted rack.
Hope this helps.