The idea of the mirror tool was a great one. The feature had quite a bit of depth but its implementation was not very user friendly. Over the years it has not been updated (to work nicely with partial measure selections, for example) and the "mirror" is easily broken making it more trouble than it's currently worth. It's now an unusable, broken relic. This, I believe, explains why people don't use it more. If it were user friendly and not as fragile it would be applicable to a multitude of engraving situations.
Most ensemble music over the last 1000 years contains a lot of doubling and/or parallel motion. If a user could program in that doubling with enough flexibility (octave/out-of-range changes, movement in 3rds or 6ths, etc.) then any correction made to one part will automatically be reflected in the other. E.g., Change the shape of a run of 16ths in the violins and it automatically gets updated in the flutes, clarinets, oboes, tuba, xylophone and whoever else is doubling that line - even if it's not at the octave.