"A welded joint can transmit torque through the joint; a screw connection cannot transmit any torque. Hence, an integrated rack can handle lateral loads such as the bags wagging left to right."

I'm no engineer, but I don't see that bolt on racks of proper construction have any problem resisting bags wagging left to right.

"The tubing can be downsized because the loop over the rear wheel does not have to be as strong."

But this is not what I have observed on the touring type bikes that attempt this:

http://www.bikebrothers.co.uk/ianhib...1_files/06.jpg

http://www.peterwhitecycles.com/tout-terrain.asp

As far as improving rack attachment is concerned there are many tested ways that go beyond the current fasteners without the lack of flexibilty of welding/brazing a rack on. Not all of these use bolts in the conventional maner, they may support the weight of the rack directly with the bolt merely securing a ferule etc... I still think that the most practical approach is to use upgraded bolts and fittings since this nips the problem in the bud, but if there is some disaster a resupply rack can be quickly fitted.