Hi Jo-anne,
We now have a Toyota Landcruiser but I have only ever used roof bars for carrying up to 5 bikes. I have seen too many people struggling to get to boot/back door with them on the back and restricting the turning ability of the outfit when hitched up, not to mention the increased weight on the rear suspension. On teh roof they are out of the way until you need them, spread teh load over the whole vehicle, can be easily secured and don't interfere with the towing.
Alan