Now I am really curious. While, I can imagine a scenario when this would be a useful feature, I can't see it being used often. In what situations are you tilting and shifting at other than right angles to each other?
Say you want to take a photo of a building, from a corner point looking along one side.
Something like this, that I just grabbed from a google search for 'building':
Now, you want to shift Up, because that will make all the vertical lines look parallel in the photo.
But you want to tilt to the left, because your plane of (where you want the) focus is near on the left side, far on the right side.
That's why you'd want to have tile and shift at 90 degrees to each other. But for other shots, you'd want them in the same direction, which is why the 'new way' of TS-E 17 and 24L is the best way (or like hartblei were making Superrotators 20 years ago, there's a reason they chose that name, you can tilt/shift in any direction you want)