I've tanked all the way up to 73 as Unholy. Obviously I use a lot of AOE when I'm tanking trash, between my own DnD and my diseases being spread with pestilence, then popping Unholy Blight.
I've had several DK's drop DnD 3-4 seconds after the mobs got to me and were standing on my own DnD, just as I was putting up pestilence. I check most of them to see if they have the reduced threat talent from blood, then I watch to see if they are pulling aggro. If they aren't, I just let it go. If they are, I whisper the healer not to heal them and ensure that the mob doesn't go for the healer once they are done with the idiot.
There are quite a few experienced players out there, capable of using an AoE mechanism like DnD on larger trash pulls to put out more DPS overall. (Obviously it's retarded to use it as DPS on a boss fight unless adds are out and the tank for some reason can't manage them all and they are going for the healer)
Unfortunately, just as many idiots are out there thinking that just because they are in blood and not frost, that the move won't ever pull off the tank. Most of these idiots are the same ones who drop DnD, then attack out of the normal kill order, ending up pulling off the tank.
I'm also sure that it's different for tanks of different specs who might have less AoE damage going out to all the mobs in the pull. Between DnD and my 3 diseases running on all the mobs, as well as UB, I have had people nuke targets out of order and still not pull off me before the mobs died.
Mostly what it boils down to is idiots. If they are, they aren't any good anyway, if they aren't, well tell them to stop acting like it.
BTW: The DnD glyph causes them to cower, not flee.