I disagree with that. A friend plays a 12 with us sometimes, and she does not spend a lot of time tuning. And I like the sound of a 12-string. It's like crystals, or a waterfall, somehow.

No instrument is played perfectly in tune. An instrument in perfect tune sounds like a computer beep.

As for John Denver, he was quite young at the time. I suspect that hoisting the guitar high on his shoulder (so he could press his ear against it) was a great way to get attention from all the girls.