Do you like spicy food?
I don't but for some reason a little bit of pepper sometimes makes some dull dishes taste better. Sometimes not.
Mixtures on the organ also add spiciness to the sound of the principal chorus.
Many organists know the mixtures usually consist of a few ranks of pipes at octave and fifth pitch level (4', 2 2/3', 2', 1 1/3', 1' etc.) Sometimes the row of the pipes break and the highest one goes down an octave to make the pitch less piercing (not so in progressive mixtures).
Did you know some regions had mixtures with the tierce pipes?
In Central Germany they definitely had them.
So if you want to play Bach with the sounds closer to what he had in mind, you need tierce pipes in mixtures.
It's not the same as adding a 1 3/5' sound to the full principal chorus registration, though.
But it's better than nothing, in my opinion.
When I added the tierce stop for Piece d'Orgue, BWV 572, Ausra initially resisted the idea because it really spices up things. But she soon agreed it's more colorful this way.
What do you think?