I'm having some difficulty in seeing the purpose
of providing this MIDI interface.
It's wicked fun. A use case that I can vouch for myself is using such
retuning as a crude single-voice harmonizer. I'll take a vocal line
and just play MIDI notes along with it to hear how it'd sound, and
when I come up with a harmony that works I'll bounce it and play it
alongside the main vocals. It opens up the possibility of creative
off-label use of pitch correctors, and I can say I would definitely
use it.
-w