It is no secret that only elite units of the Iranian infantry were capable of holding their own in a fight. But why, oh why, did they not develop something more effective than the single rank spearmen + load of archers for their foot, or simply dedicate themselves to the cavalry?
The archers were quite useless. The only use for missiles is to harass the enemy a bit before the melee ensues. The overly fixiation on the foot archers resulted in catastrophic failures when facing anything other than more archers. A single rank of spearmen is simply not enough, especially if those spearmen are not trained and equipped properly. In fact, sometimes even the front rank carried only bows, setting up shields on rests. Which means 90% of the Achaemenid infantry force could be neutralized by simply issuing shields to one's own troops.
To sum up - the crusty-archer formation was inefficient and had little tactical use. More infantry or more cavalry would have made the army more efficient.
Why did they use it?