Whistling was actually accidentally removed a few patches ago... ah, those were the days. But then this foolish dev fixed it back to normal. I liked it, a lot. People were leaving horses alive cause they knew a downed enemy cav from the other side of the map wouldn't be able to get the horse. At the moment when I see a stray horse I quickly check if there's allied guy coming for it, if not then kill it. Whistle fights over horses, how retarded is that?