I dont think auto-cast is so silly.
If you are the first attacking, the target is unable to get the first hit, or buff themselves. One might also target, then wait till they start casting, then attack (at least in dungions).
Devour Magic is more valuable to me, at least as a Demonology warlock. It knocks-down shields and other buffs, and keeps my felhunter going for a long time. It is very difficult to kill my character with magick, and can only be done if there is a group on me; my felhunter's resistances are each well over 200, so using magick against it is futile.