I never liked behind the neck pulldowns, but now that Atomic brings it up, I can't do shoulder presses behind the neck because it puts too much strain on my upper back and neck. Anything above 135 amd I can't move my neck for a week. So I do all my overhead presses to the front. My guess is alot of people will have the same problem with keeping your arms back during the pulldowns.
There was just a thread about people telling us not to do squats and deadlifts because they're bad on your knees and back. Do any of us really care what someone else says? If you like the exercise, if it works to make you bigger, stronger, faster, and it doesn't give you trouble anywhere else, then hit it.