The purpose of giving archers a melee autoattack is to give them some means of defending themselves when the mob loses bind and charges up close.
The real purpose of giving archer melee auto attack is because they are trying to shoehorn in code and this was their only fast solution.

The reason you just stated is ridiculous. We should stay out of range of a mob, hence being ranged DPS. If a mobs in our face we are pulling too much enmity and should let a tank to take it or bind and run.