I can understand the futility in having your warnings go unheeded. As for myself, I can usually tell when the whistling is part of a random command spam, and when it (rarely) is something else. When in "archer mode", zoomed in and not paying attention to my surroundings, something like a whistle would at least remind me that there's a world around me and I should probably take a look.
As a side note, this thread has had a few reads. To those of you reading this, lets all try, at least for a week or two, to use the whistle to warn our archer friends. Who knows? Maybe it will catch on. Rebecka Black certainly did, and what she did was a far worse idea than what I'm proposing...