Worries about twisting have come up when I have talked with others about this.
Just for ease of discussion, I am going to call the long axis of the tubes the x axis, the axis across the tubes the y axis, and elevation z.
If you try and move the cross bar in the x-y axis, it is restrained by two things. First, because the curve matches the tube, the tube itself resists any movement in the x-y plane. This is because as you try and rotate the cross bar, the effective cross section of the tube gets bigger. Second, the grommet strips are fairly rigid in maintaining a 90 degree orientation of the cross bar to the tube.
Now, for twisting in the y-z plane, the grommet strip psychically restricts the cross bar from rotating toward the outside of the boat. As for the cross bar rotating toward the inside of the tube, friction from the grommet strip and the force acting on the far tube prevent this.
Twisting in the x-z plane resisted solely by the the grommet strip physically restraining the cross bar.
Here is a look at the grommet strip:
The lacing on the cross bar holds the cross bar totally rigid (for an inflatable) basically because it is really tight. It is easy to do this by deflating the tube lacing in the bar and then reflating the tube. When the tube is tight, I could not get the bar to move relative to the cat tube. This is with a buddy holding the tube and me using the cross bar as a 4 foot lever arm. With 3 cross bars spanning the tubes, I think the boat would be pretty stiff. With currently frame designs, the rigidity of tube is simply not used because the cam straps cannot transfer a moment. With this design, the tube because a structural piece of the frame. For an example of the stiffness of inflated tubes, think of the overhead piece on a creature craft.
As for why not using inflatable tubes? I guess that works, but the boats I have seen out there that are out there that are inflatable are heavy and small. My goal is a 12' cat that comes in at 40lb total. Also, it has to split into 2 packages. Plus, I think this design would have the flexibility to be rowed, or paddled without purchasing a separate frame.
Plus, its just fun to build stuff.