Backwards compatibility aside, I think this would be a good thing even with the current interpolation approach. In particular it would allow (at least by default) enforcement of the keyframes themselves being normalized, which generally makes quaternion rotations easier to work with.
Back when working on Big Buck Bunny, Blender actually auto-normalized quaternions when inserting keys, which made it feasible to essentially “sculpt” the rotation paths and ease-in-ease-out by just inserting more keys and moving them around in the dopesheet. That workflow, while not impossible now, certainly isn’t nearly as effective as it used to be in 2.49 and earlier.