"IMO, the look of the swing should be changed only if it's related to a flaw that causes faulty flight/contact. There are a lot of pretty swings that don't do anything, and a lot of butt-ugly swings that produce the prettiest shots you'll ever see. To me, a good instructor/smart golfer would do well to leave it alone if it's working, rather than changing things for aesthetic purposes."
In my many decades of playing golf, I have seen way more swings (pretty or not, mostly not!) that did not produce consistently good shots. My point is that I think more instructors change a student's swing because the student is producing poor shots. Almost has to be true because there really are not that many pretty swings out there as a percentage of all golf swings.
Just my entirely subjective view.
I agree with your point, and I know that instructors have their hands full with unproductive swings; didn't mean to imply otherwise. My point is just that if you're hitting the ball well and know where it's going, don't change the swing just to make it look better. Wulsy's post touched on something that I think about from time to time, especially since I've known good players who've gone after a swing change simply because of the way their swing looks, rather that how it performs. Yeah, pretty is nice, but performance is what counts.
I've had people tell me on the range or the course (even a pretty good instructor) tell me that I had a good looking swing/action; I've even had people stop to watch me hit. At the time, I had yet to break 100. I would have gladly traded my "nice looking" swing for "an octopus falling out of a tree", if it had me threatening par every once in a while.
Like Birdie said, results are the ultimate. And while I love to look at and admire a great looking swing, I'm not going to automatically assume that a great looking swing = a great ballstriker. I've seen enough in my short golfing life to know that isn't always the case, and if I had a say, I wouldn't tell someone to change if the only reason was aesthetics.