////The idea of closing the circle of fifths by 12-tone equal temperament has exactly as much rationale in traditional music as closing it by 19-tone, 31-tone or 53-tone ET, i.e. none whatever.////

It sure does. They've built 31-tone keyboards and they've discovered that while it sweetens intervals currently a bit dissonant in 12-tone, it made other intervals even more dissonant, In fact, the perfect 5th becomes more dissonant in 19-tone and since it plays such an important role in music, we don't want to worsen it. Moreover, the keys had to be narrower otherwise the keyboard would be too long to play which makes it harder to play. They tried switching the positions of the black and white keys which would have been a good solution except keyboardists simply didn't like it. 19-tone is cumbersome and 31-tone would be even more so. They're harder to play, harder to build, harder to tune.

12-tone seems to be the limit that the human mind and body has a natural aptitude for. It allows us to modulate the diatonic scale through twelve keys and gives enough physical space to do it in. It works very well and the secret to understanding how to use it most effectively use it is the circle of 5ths. It's a beautiful scheme.

////For folk idioms, you simply don't need to know ET ever existed.////

That's like saying to understand alchemy you don't need to know a thing about chemistry. Or to understand the humors and the biles you don't need to know a thing about modern medicine. The truth is, your understanding of alchemy is deepened greatly with an understanding of chemistry. Understanding modern medicine helps us to appreciate what the old study of humors and biles was aiming for.

////It's a piece of 19th century art music theory which is great for understanding Wagner, Schoenberg and Duke Ellington, [and virtually any other artist you can possibly name--Josepp] and completely pointless for anything far outside that range of idioms. ////

Actually, 12-TET was aspired to in Bach's time, they just didn't have a way to gauge it precisely. And the man credited with coming up with the value of the square root of 2 as the measure of a half-step was a Chinese theoretician named Zhu Zaiyu in 1584 proving that 12-TET was being searched for long before the 19th century and outside of Europe. Going back to Ancient Greece, the god of music was Apollo who was also called Pythias. In Greek isopsephia, where each letter has a numerical value, Apollon adds up to 1059 and Pythias to 1061. The value of the square root of 2 is approximately 1.0595. Coincidence, no doubt.

////It doesn't make you play any better,////

Neither does taking lessons but I'd rather take lessons than learn on my own.

////doesn't add any understanding of the music you're dealing with,////

I flat out disagree with that. It adds a great deal of understanding of the music you're dealing with. It is the very secret of how to utilize that music in the most effective way.

////and it seems it blocks off your perceptions so you can't even hear the sound of a mouth organ right////

I agree with Don here. You're talking differences too minute for most people to care.