Yes, they already have such a simple thing. And I say YES to "autocasting", and hear me out on why.
First of all, there's the whole matter of choice, the player can override these commands at any time, and most of them are far, far too complex to do for a human player, yet are unrealistic without. For example, in a mass battle, are you going to select your archers one by one when swordmen come near so they can switch to a melee attack? No! This just lets you use that melee attack without having the impossible task of switching. Any battle with more than 5 units on each side would become too overwhelming.
This does NOT mean the CPU is playing for you, it just automatically switches the attack based on idiotic computer formulas. It has no idea what the battle is, and only hopes that in most situations, it's the right choice. There's still micromanagement by manually controlling your unit. For example, you might tell your archer to manually chase a fleeing foe with his melee attack (using a ranged attack on a fleeing foe is unlikely to hit, and "autocast" will not switch here). There's still plenty of micromanagement.
This isn't meant for the tiny battles that you might see when the enemy sends a scout, it's meant for the large battles that make up the core of the game, where there's more than enough micromanagement just sending your units to attack targets, not even having to worry about multiple attacks. The "autocast" doesn't take away micromanagement and strategy from the game, it adds it! Why do you so oppose it anyway? After all, without autoswitching, there's no point in giving an archer a melee attack since it will be impossible to use really, but with it, there's a reason to give it that melee attack, as the CPU and player can actually make a use of it then. Further than that all depends on the modder.
And going back to the concept of choice, suppose there's an INI option to enable/disable it. Granted, you'd be weakened/less realistic with it off, but if you're too stubborn to accept the concept, well, you turned it off, can't complain, huh?