The reason we were taught that can be seen in the pic that gzg posted of the sergeant; while he does have all his skin covered, there are still definitely "shine" areas that are problematic -- his nose and cheekbone areas specifically. And for those who would use him as a role model just because he's wearing the uniform... well, it's like any other profession: You get taught how to do it and after a while start deviating into your own thing.

Take it from the guy who went to sniper school; there's a reason why it's taught the way it is. Again, this is all a moot point probably when it comes to being 20' up and your quarry is a deer. A few of us are just letting you know what the proper answer is from a technically sound standpoint.