Keeping the support arm out in front also minimizes elastic tension in the shoulder area that would cause the whole swing to be 'rubberbanded'. Try for yourself, stand up straight, put your left arm straight out to the left and then swing your right arm and see what sort of max swing you can achieve. Now move the left arm out in front of you and let your shoulder area move with the swing, quite a lot easier and not nearly as much stress on your joints and ligaments.
Back in the day, square and straight out was the best knowledge available and worked for the style and equipment available. It still does in some circumstances, but the 'new' way is a lot more forgiving once you get used to it.