Why don't I hear about Equal vs. Just tuning on other instruments
that play both single notes and chords? You never hear guitar or piano
players discussing this. Why only harp?
IF you decided to Just tune a piano, it could only be done for play in one
key.
Where the instruments have a built-in chromatic scale (e.g. keyboard and
fretted instruments) AND they are played in various keys, only equal
temperament will work. In these instruments, the diatonic scale for each
key is mapped over the same built-in chromatic scale but at different
starting "do" notes. If the temperament were not equal, the intervals
between notes in a chord would be different for each key.
Analogy: A gear whose teeth are evenly spaced will mesh with other gears of
the same pitch even if they have different numbers of teeth. A gear with
unevenly spaced teeth would only mesh with one other gear specifically
designed as its mate.
Each diatonic harmonica has its own key and so can be just tuned for play in
that key.
Chromatic harmonicas must be equal tuned to be played in different keys on
the same instrument.
Human voices and unfretted stringed instruments such as violins may be
played in just tuning because they do not have discrete pitches, that is,
the pitches of the notes are determined by the human ear. This is what gives
a-capella vocal music such as barbershop and string quartets their
wonderfully appealing chords.
Guitars can be retuned quickly and easily in anticipation of the key to be
used. You sometimes see guitarists retune for every key. Obviously you
cannot do that on a piano or chromatic harmonica.