Completely agree. I think riding horses with issues can cause more problems than it teaches skills. There's a difference between learning how to ride a sensitive horse and learning to ride a horse with a dirty stop.
As for quirky horse teaching people to be riders....well, I do agree with that, too, to some extent, but quirky horses also teach people to be defensive, nervous, or scared. I consider myself a pretty decent rider, and I've ridden a lot of quirky horses over the years...and now I'm struggling to break a lot of defensive and nervous habits because I am now sitting on a truly GOOD horse. I have mixed feelings about dealing with quirky to "teach".
To me this often happens when there's ego involved (not directing this at the OP). Rider feels she/he "should" be able to make the horse work and that it reflects badly on her/him if they get a different horse.
I don't quite understand the mentality of "you owe this horse the chance to figure it all out." WHY does she owe this horse that? If they don't get along, don't click, don't want to do the same thing, don't trust each other, WHATEVER, all she owes him is the chance to find someone who will appreciate him and give him a job he can and wants to do.