So I'm confused, is the point of this thread to educate people that a DK is going to take awhile to build up AoE threat or are you trying to say that a particular talent like morbidity is not worth taking?
While the math and everything is good, it doesn't take one important thing into consideration, and that is snap aggro. The reason tanks take Morbidity is so that their DND is 15 second cooldown instead of 30, and the reason DND is used is because it does AoE damage right off the bat, and generates high threat, and gets mobs attention right off the bat. Mobs aren't going to wait for you to Icy Touch, plague strike, pestilence, and then wait 3 seconds for a tick before they run to start beating people up.
In the end, DND is a staple for any DK in a AoE tanking situation.
Again, I could just be confused what you are trying to say here, because I think maybe I read you saying 'DKs don't just use Death and Decay to AoE tank.'
Maybe a little clarity is in order.