As someone who did not play as DK, but played a lot with one, well, probably the best co-tank one could get, each spec could tank for a specific role - trash/aoe, bosses, you name it.
Plus they were pretty good DPS too, again, all specs could be used for different scenarios.

So I was quite dissapointed when Blood was the only tanking spec left. Of course, balancing it was probably pretty much impossible, so it was logical to see it all go, but still.