Harmtouch is not meant to be one of the core damage skills for Dreadknight. harmtouch is one of those skills (like lay hands) where if you get into the situation that it is desperately needed, you can get an instant burst off. As Kainzo said, Deathknight is a tank class, therefore the damage output should not be as reliable as a class such as wizard.
I see where you are coming from on this thread, as I have felt the same way before about lay hands, but you need to realize that harm touch is a kind of save your ass ability; it can be used to burst someone down, to either finish them off, or scare them away. The long cool down compensates for the fact that it is a very powerful ability on a tank, and should not be used in the core rotation for Dreadknight.