Is Dave Scott reading this forum?
Werner, imagine having a 40 x 100 cm plank en pulling that plank through the water while rotating that plank back and forth at the same time. Can you imagine how much force that takes?
Now imagine that plank being flexible so it can twist a bit while you rotate and pull it throug the water. Is this costing more or less energy? The flexible plank is easier to pull through the water while rotatiing.
A plank is stupid and a body can be smarter than a flexible plank. A smart flexible body is much more eficient than a dead treetrunc in combined propulsion and drag optimisation.(dolphin)
Only when the body is drawn through the water without rotation a stiff body held in optimal shape is better.
And a stiff body is better than a wrong moving body offcourse.
Maybe its also because TI likes the body traveling forward statically on the edge for a long time with short shifts to the other edge. Glide on edge--shift---glide on edge.
During that glide on edge its not effective to have your body twisted. Thats where your idea is coming from that keeping everything in line is better from a streamline perspective.
If you like the continuous roll model better, then the continuous change of twistangle is more effective (for optimal propulsion drag compromise). Thats my favorite model , although the difference its not completely black and white.