Since you have an upper and lower link only on one side (the other side typically only has a lower, but it is possible to run only an upper, and omit the lower), the tube on that side is the only thing keeping the axle from spinning. Think about the force an axle endures. When power is applied to the axle, the tires rotate in relation to the axle housing. If there were no suspension to control the pinion angle the axle would just spin around, instead of moving forward. With a three link as he's talking about, the only thing that stops the axle from spinning is an upper and lower link, on one side of the axle. The plug welds connecting that side tube to the housing have to be strong enough to resist the twisting force applied, since there is nothing on the other side of the axle to help control this force. On my last buggy I had a front 3 link of this design, and a narrowed high-pinion 60. Since it was a front suspension, and a 60, I thought I could get away with it. I welded the tubes to the housing just for insurance but even that was not enough. It spun the tube after a couple years.
It's tough for me to put the concept into words, but did any of that make sense to you?