To me, on a JK, a roof rack is a necessity for my system. I use an RTT, and an awning, and I like to go to places where I will have overhead clearance but there's hairpin turns or tight corners where a trailer simply could not go (at least not with anything resembling ease). I think 99% of the terrain in North America fits this description, and for the other 1%....that's why I have a motorbike
Take a gander at xoverland's video and the fun they had with their trailer; check out about 8 minutes, 38 seconds in for the trailer stuff:
A roof rack'd vehicle still would have a rough go of it in that terrain, but the trailer made it nearly impossible. The very thing that necessitates a trailer and a rack in their case (lots of people) is the main reason they are able to get around it -- if I was trying to do that on my own or just me and my wife, it'd be dangerous and exhausting. Please note this isn't a slight of the xoverland guys choice of using a trailer -- I love their series and thoroughly enjoy their videos. Most people would give up in their situation, but they got through it, and as I said, with the number of people in their crew a trailer makes sense.
As for fuel economy, we don't lose much -- maybe 1 litre/100 kilometers difference, which on a tank, means we are burning about $5.00 worth of gas lost to wind resistance. That number is important because, for many:
Gobi Stealth Roofrack -- about $2,000
A Trailer with comparable utility (i.e. an off-road worthy trailer) -- Between $3,000 and $5,000 or more.
So I'm looking at getting on the modest end, 100,000 kilometers of travel before the fuel economy difference actually adds up. My math on that is as follows:
(Based off prices I could find online, and the following numbers: I get about 500km to a tank of gas. My rack, I estimate, costs me about 1 liter per 100 kilometers, or in other words, I effectively waste 5 litres on a tank of gas just pushing wind resistance. At an approximate $1.00/litre, that's $5/tank of gas wasted pushing wind.
$1000 (price differential, rack vs. trailer) / $5 (wasted per tank) = 200. It would take me 200 tanks of gas to equal the price differential of a trailer vs. a rack.
200 tanks of gas, at 500 kilometres to a tank = 100,000 kms.
As for weight and COG arguments, that is a major concern with any tall 4WD vehicle, and while a few hundred pounds up high can make a difference, at trail speeds that isn't as big as some make it out to be. High COG becomes a much bigger issue at highway speeds than at the typical sub 50km/hr trail speeds. On gravel track where you can get a good clip, a sudden turn maneuver amplifies the weight up top and can send you for a tumble. Slow down = no problem, and you'll see more things
. On technical stuff, where you'd be going slowly, your rollover angle doesn't change that much by having a few pounds up top.
There's a lot of physics involved that I'm reasonably familiar with -- 300 pounds on roof rack can act as a lever, and may exert the force of 3 times or more that weight on actually flipping the vehicle -- so make no mistake, heavy stuff should be down low! But a few crates full of clothes or food is really not a big deal. Most roof racks have a dynamic load rating of 300-500 lbs. That's a fair bit of stuff; Roof racks are best in my opinion for the bulky, lightweight items like, for example, a tent (rooftop or otherwise).
On that score, for a RTT you need a rack for the JK, unless you want to go with the J30 but to me, as much as I love it, they preserve one of my biggest gripes about the JK: A fiberglass top. There's not much you can do to it without it cracking on you.
I'd really love to see what these guys could do to the JK. Lightweight, strong enough to bolt things to, and they don't add a ton of height and look practically stock.
http://www.alu-cab.co.za/