I feel it would not be a very enjoyable feature to include.

At the moment we can pace our own attack speed by use of Stamina. You either blow through it, making some quick attacks, or you save up stamina by not unleashing as many standard attacks so you can use that stamina on more special attacks right after each other.

This adds a sense of tactics, planning ahead and in general makes you feel more like you're the one performing the actions. I personally feel that Auto-Attack would severely hinder that. In general, it would mean that the Stamina feature this game has would cease to exist. There's very little fun to be had in just engaging an opponent and having everything be done for you, regardless of it being more battle efficient.