This is a good case of bring the player not the class. If the person or persons whom you want to bring prefer to tank as a DK then let them. If you are looking for optimal grouping in a 10 man... Well good luck, and bring whatever you can use in the most situations. I.E buffs, rolls with off spec etc. For ICC, at least once you need to bring a Bear tank... cause you have to witness a bear flying around with a rocket pack on his but.
Ohh and you are a prot palie so if you have melee dps then a war or Dk would be nice as off tank main tank. They both have debuffs that help. The war brings there shouts and sunders which help with dps, the DK brings frost fever which helps reduce some damage to the tanks.