If there is no other answer in sight, I believe, that this is to make sure that the optimizer in the compiler will not throw it all away. It will know that input pins are "volatile", so they must be read in again each time, and cannot be "cached" in a register.
But after all, I believe, that this is a kind of "bug" in wiring_shift. If there is no documentation about the shift speed, it is not really a bug, but as other libraries are using it to connect to existing HW, the shift speed should probably stay the same as in 16 MHz arduinos.
I will probably need it soon, and if my concerns get confirmed, I will give feedback again to the forum.