I once tried a squad of 7 caelian scouts equipped with ethereal crossbows. The 7 where spread out between ordinary archers for protection and accompanied bigger armies, usually with a mage casting wind guide. I did not notice that they presented a great help.
(The scouts were merely scripted to fire without any specific target.) Maybe I did something wrong or just had bad luck. Maybe I should have made a large squad, but it is too much trouble to create and move them around (since they are all commanders).