Good you noticed that, Sarkiss. Being an archer myself, I would say that we can safely assume that arm guards are as old as archery itself. Shooting without it could lead to severe injury to your forearm, while making an effective armguard is extremely easy. You could even do that by wrapping the arm in a piece of cloth. Anyway, whoever could invest the energy, time and resouces to procure a bow and arrows would certainly be able to protect oneself by some sort of arm guard. Therefore I would support the case for giving all archer units something like that.

Cute Wolf - it is NOT a "little pain" easily endured by a hardened veteran. Repeated hits by the bowstring could even sever your skin or lead to painful immflamation of sinews in the forearm. Hard skin is no protection. You need something capable to absorb the hit and disperse it onto larger area. Tough leather is perfectly sufficient for that and easily obtained.