That's the problem! When someone is using "heavy" weights it's WAY TOO EASY to lose form then POP - there goes a shoulder. Kinda like how often do you see elbows flare out when someone is having difficulty with bench press (which throws the stress on shoulders) - again, asking for problems.
I guess you can say FORM is important in any exercise. BUT - because of the unnatural position of behind-the-neck and concentration needed to overcome it, why push it when there's so many other good shoulder exercises?