Historically missile units did not seem to be as effective against unarmoured units as in EB, but Tacitus Annals tells us of germanicus' campaign to re-conquer the province lost to Varus, and archers are mentioned here and there as part of the army.
However, there is another point called national military doctrine. You might think that the Romans would drop legions and go all-archer, but Rome was a "heavy-infantry" military doctrine state, just like the various Hellenes were if in a different way. That was the mainstay of their armies and their tactics and training as well as thinking was built around it. Even in the late Empire when they confronted Cataphracts and HA they only adjusted and created their own (Cataphracti and Clibinarii at least), but still the heavy infantry was the core of the Roman Army until at least Chalons where it is said they did nothing but cover under their shields. Roman doctrine was heavy infantry, probably determined by their geoploitical location.
Bookmarks