Yep, which is what I infer of Oleander saying "if you have a unit with decent morale winning a fight." Both units will take the morale hit, the difference being that your guys can afford it, while the enemy cannot. And with Whistling Shot, you don't take significant friendly fire, since the arrows are almost harmless.

Bottom line; low-end archers can hasten the end of a fight that your guys are already winning, by making the enemy break that much quicker. But they're not going to change the morale balance and turn around a fight that you're currently losing.