Archers and slingers are quite useful .
That annoying skirmisher cavalry unit for example ; don't waste your expensive cavalry , shoot the mongrel !

Enemy skirmish infantry in the way of your mercenary Gallic troops eye-watering charge ? Not if your archers and slingers kill them first ! {Note , Pelstati and similar have reasonable armour , you need slingers to get the best results on them} Now your charge hits their expensive troops .

If you fight horsearchers , you have zero chance without lots of good archers {mounted or foot} and slingers for the armoured ones !

Tired of those Phantom generals {you know "the General who cannot die !"} ? Try whittling his bodyguard {and maybe some of his own hitpoints} with slingers before you try to melee him .

Want to draw the enemy onto ground of your choosing ? Archers and slingers are long range skirmishers and baiting the enemy into moving unwisely is one of the main reasons they were use in real life .

Got an Elephant problem {or scythed chariot for that matter} ? Not if you have archers and especially slingers !

A liberal showering with fire arrows can greatly lower enemy moral .




There is lots of uses for those units . Given you sound like you would only use them sparingly , you should probably {playing Roman} hire Mithroporoi Toxotai Kretikoi {decent archers} and Iaosatae {the Celtic slingers someone mentioned above} . One unit of each is plenty in most situations .