Roof Rack luggage systems are great for getting the bulky items out of the cabin of the vehicle. With additional accessories such as weather-proof bags and boxes you don't have to worry about exposing items to the elements so you can even store your tent and pillows up on the roof for a quick camping setup.
The obvious disadvantage is the added height a set of roof racks or luggage tray will add to your vehicle. While this may not pose an obvious problem in the bush you will certainly have to check clearance into the
home garage and underground carparks.
However, most importantly, is the safety and handling aspect of increasing the height of a weight load to your vehicle. The vehicle's centre of gravity will be increased by adding a loaded roof rack, which can increase vehicle sway and degrade offroad handling and performance.
In comparison to towing a trailer however, a roof rack system is much cheaper and probably a lot less hassle. In the end, the choice is personal and you will have to consider all the pros and cons yourself.