I have to agree, blood doesn't hold a candle to frost if the competition is over quickly obtaining aoe threat and holding onto it, however, that isn't much in terms of raid tanking.
In my opinon, and i'll try to type this so it doesn't look like pure qq, a frost dk basically adds up to a very gimped prot pally in terms of handling aoe. It's not something I enjoy admitting. I tank along side a prot pally, and have done so since 3.1. We're both very familiar with our strengths and weaknesses. I won't go into the laundry list of encounters he currently has an edge on me for, but the MOST obvious trump card of the paladin in current (ICC) raid content is threat, and aoe is no diffirent. Our paladin can dump more burst threat on 3 targets than i can on one with tricks or md in a blood spec.
I've stuck with blood because it plays to the one relative strong point that I think we have as DKs. That strength is being a self-sustaining health sponge. I have given frost (and unholy) a more than fair shake in ICC and TOGC since 3.3. Both specs show some real promise, and I would never accuse either of being obsolete, non-viable, or without use. With that being said, I believe blood offers us more options for current and upcoming encounters.
The first wing of ICC doesn't really have anything for us. We're relatively sub-par tanks against fast physical damage (yes, even as frost) without a block mechanic. Death strike helps to level off your heath losses, and makes healing on the dk more predictable than a frost or unholy dk who is relying more on avoidance or pre-emptive cooldowns to redux the damage before it happens. Against steady hitters like overlord, airship captain, and marrowgar I see blood as the superior spec because of this, not to mention wotn that has saved my butt on several occasions.
So, just to recap where i'm goin with this post: Blood has a weakness in aoe threat, that much cannot be denied. Snap threat in particular here is probably the weakest in the game. But, that is something you have to live with, learn to adjust to, or have a co-tank assist you with if you want to be the most effective in other tanking duties (namely tanking those big nasty raid bosses). Frost might look very appealing right now with a bit more passive mitigation, but when push comes to shove blood is still going to be standard issue for progression. There just isn't another spec that offers universally high eh, cooldowns, and (magic and physical) capability in terms of soaking damage, plenty of single target threat, and a bit of protection from overkill (wotn).
So as I stated from the beginning, let your warriors, paladins, or druids get the aoe packs that call for snap threat (unless you really must do that job, then spec frost), and perform roles that work to your (relative) strengths. DK's may well be a tad sub-par across the board, but there is no reason for the underdog to do the job he is the least suited for just because the other tank doesn't want the inglorious job of tanking adds.