Depends on the situation I guess, and what you mean with efficiency.
If you have the support for a DD tank and want the extra damage, sure, but if we're talking about tanking efficiency in terms of max survivability and max enmity, which really shines in low low man situations/holding with no elegy or slow, or when rushing to a HNM pop, DD tank isn't feasable/efficient.
Other example is HQ worm, RDM tank just pwns that bitch, and I don't think DD tank is feasable let alone efficient (phys absorb + gale pwnage).
http://i50.tinypic.com/vflat.png
7+1 (BLM mule) kill in 45mins with no problems at all, surely was one of my favourite accomplishments.