I appreciate the devil's advocacy as it makes the thread informative and useful. I am now a little confused though on how a full platform rack is going to be stronger and lighter than multiple crossbars arranged close together such as this rack or the new front runner slimline kits.
If you have a platform that is made of a expanded steel mesh or thin material, it is only going to be really strong on the areas directly on top of the main beams. Just like floor of a house. The beams on the sides support the joists, the joists support the flooring. Sections of the flooring directly on top of the joist are strong, but the further you move from the joist, the weaker it gets because the flooring is generally a thinner material. So the middle point between the two joists is the weakest point.
I would assume these racks have a similar structure (Although I haven't seen them) This method using 6 "joists" with flooring or platform on top would be lighter than a floor made of 18 joists all back to back, but it certainly wouldn't be stronger, unless I am off my rocker. Now if you even out the scenario and you have a rack with 6 joists and flooring vs. 6 joists only, then yes the flooring helps reinforce the the joists and distributes the load capacity and it is stronger, but it will also be heavier because you are adding the flooring.
I think a compromise between the two to have even strength and weight would be to have 6 main "joists" or crossbars, and fill in the gaps with a lighter, secondary "joist" or crossbar. Then you effectively create a full platform while reducing the weakness of the points between the heavy joists.
Now with that whole ramble done, I think that when all is said and done a roof rack shouldn't be used for super heavy items anyway. Some items it makes sense, like a RTT or lighter cargo items, but when I see a vehicle with a RTT, 4 jerry cans, 2 huge pelican cases, a hi-lift, and a spare tire all crammed up there I get worried. The behavior of the vehicle can change so much with that much weight that it can be dangerous.