AqD

Parthians did have very good bows, even better than scythian ones (used by roman auxiliary archers). BTW huns are not really as same as Xiongnu - their appearances and cultures were very different and the only thing that connects them is the vast steppe in which nobody really know what happened there They did employ loose formation to avoid arrows if needed. But fighting in loose formation? Hack no. Against a few arrows their shield combined with mail armour is usually good enough; Even in the battle of Carrhae, when they're completely surrounded and unable to fight back, they didn't suffer more than a few thousands of casualty during the actual battle - and those were mostly wounded not dead. Which is not needed unless they're forced to fight in melee (which means they'd lose and die no matter what). Their saddle could absorb charge power and the lance is basically a psychological weapon meant to rout enemies. In fact the parthian/sassanian/sarmatian cataphracts should be even more effective than medieval knights, because they charged in very close square formation, and always operated together with horse archers or foot archers, whose arrows could usually force enemies to employ loose formation and become vulnerable to charge. It's just a primitive crossbow, more likely to be a hunting tool rather than real weapon. Note that crossbows alone are not very useful except for ambush, due to the painfully slow rate-of-fire. In china the crossbowmen were usually used together with archers - in Tang and Song dynasties for instance, the number is about 1:1 for foot archers and crossbowmen.

So it's a shortcut to get armies of different skills. But this causes a big problem when their original armies become less useful (in later stage of empire) - they have to rely more and more on the auxiliaries, especially cavalries. Wouldn't they wonder where their native heavy infantries can still be found useful? Why not train their people to cavalries and archers instead?

Why does roman empire/republic use auxiliaries instead of training its own people for heavy cavalry and archers? The use of barbarians in army seems to be very illogical - instead of diminishing the barbarians' battle capability, Rome actually helped them by recruiting and training auxiliary troops, which proved to be risky and a source of problems in later empire. Besides, wouldn't it be much safer for romans if they just annihilate every enemies, instead of using their previous enemies (people in non-rome provinces) to fight against other enemies?