In Pirsig's view Plato comes in for a bit of a scolding because of his disparagement of the Sophists. I found a passage in one of Owen Barfield's book that may help to restore Plato's reputation vis-a-vis Pirsig:
"…The more common a word is and the simpler its meaning, the bolder very likely is the original thought which it contains and the more intense the intellectual or poetic effort which went to its making. Thus, the word quality is used by most educated people every day of their lives, yet in order that we should have this simple word Plato had to make the tremendous effort (it is one of the most exhausting which man is called on to exert) of turning a vague feeling into a clear thought. He invented a new word,= ‘poiotes,’ ‘what-ness,’ as we might say, or ‘of-what-kind-ness,’ and Cicero translated it by the Latin ‘qualitas,’ from ‘qualis.’ Language becomes a different thing for us altogether if we can make ourselves realize, can even make ourselves feel how every time the word quality is used, say upon a label in a shop window, that creative effort by Plato comes into play again. Nor is the acquisition of such a feeling a waste of time; for once we have made it our own, it circulates like blood through the whole of literature and life about us…"
From Barfield's History in English Words, p. 18-19.
Several things about this: first, Quality in the sense of "of what kind or character" does not pertain to Pirsig's criticism of the Platonic Idea of the Good. The definition of Quality as Excellence or Good is a different meaning of the word from Quality as the type, character, tenor or nature of something. It is important to bear these two senses of the word in mind, as for example, in the history of philosophy, in which the question of the "primary qualities" or "secondary qualities" of things became a matter of debate.
What is the "qua" in the sine qua non, mentioned earlier?
More on these matters in due course.