circumpolar doesn't make sense as an orbit. The crazy shuttle design requirement was to return 40 tonnes from a single polar orbit in 1 pass. I.e bring a spy sat back to america without flying over russia.

This means a massive body to hold an object, The earth would move under the craft as it rotated. Without being able to wait for it to rotate around you need wings for the large cross-range capability to glide back to america.

As KSP teaches us you can't put massive wings at the top of a rocket, without even bigger wings at the bottom. But that's got too much drag to get to space and so, the insane launch system that was the shuttle is invented.

They never used this ability for the entire service life of the shuttle.

It means being able to reach more kinds of orbits, which adds requirements. So does reentry capability. Why add those two sets of requirements to the same vehicle, see?