It is my understanding that DND is always the "best", but not strictly in terms of pure threat. It definitely has a convenience and assurance edge over BB. Mobs in heroics like to stand all nicely grouped up to begin with, and you don't run the risk of the healer aggroing a bunch of them because of ticking hots before you can spread your diseases.
If they didn't die so fast and longer term threat mattered more, then I wouldn't be sad to not see dnd anymore (perhaps beyond initial aggro, depending on if they're already grouped up or not). In any case, I have run with an alt tank in my guild who doesn't use dnd, and it works pretty well. If this DK in the group were doing that, I wouldn't mind. But by single target, I really mean single target. No spread diseases and Boiling Blood spam.
I think his diseases were being spread, because the healer wasn't pulling aggro each pull, but anything I tried to dps beyond 2 or 3 GCDs was definitely aggroing on me.
...and having to wait in a heroic? Just not fun!
Though a sub-10 minute Azjol-Nerub still eludes us... Anub is not at all cooperative with all that submerging.
The issue with DnD for aggro is that it takes a bit for it to ramp up. If you have really hot dps, it doesn't lock them fast enough. As Khross points out, it's better to HB them (glyphed so they are now also diseased), then OB (hope for a proc on Rime) and BB twice. From there you've pretty much locked down threat unless there is a big gear discrepancy between the tank and the dps. If it's lots of mobs that will be up for a bit while getting aoe'd down, then you could hit a Bloot Tap when the initial HB runes are back, and toss DnD down.
I've also done HB, BB then DnD as a starter rotation which works fairly well too.