I suppose this might be more efficient than normalizing the quaternion, in some contexts. There has been quite a bit written in the graphics literature about doing this stuff, so you should definitely check that out. One thing that seems like it might be a problem but isn't. If you interpolated between two antipodes on a sphere, it would pass through the origin and you couldn't normalize. But you never interpolate between antipodes with quaternions.
Antipodal points represent the same rotation.
You get a choice of points to interpolate between, so you take the closest pair available and avoid this problem.