The problem with Obliterate proccing free Howling Blasts is that there's no way to effectively control it so that you can just spam one button and make use of the free procs. If you're just weighting Obliterate then you end up with a wonky rotation.

As it stands now and on the PTR, the single target damage you do via Necrotic Strike, Howling Blast and Frost Strike is comparable to what a solo player accomplishes by managing the procs and using Obliterate.

Regarding the group damage--On live I have many games where I swap targets and my new target is at less than 50% health. Even with two healers it's damn hard to keep a person up under the 4 DK assault. No healer can effectively keep up your burst target plus the secondary/tertiary that's getting hammered by Howling Blasts. Even losing 60% of that damage on the secondary targets does not phase me--Now instead of being at 40-50% when I swap to him he'll be at 60-70? Tops? That's still *damn* good considering how much single target damage you're doing.

If I could conceive of a way to one-button mash and have it Obliterate and Howling Blast smartly I'd give it a try. As it stands the dps gain doesn't make up for the efficiency you lose by having to think about it.