It doesn't hurt to experiment, see how things are now that you know where to position toons and such.

The DK is probably the worst tank in the game, from a boxing standpoint, although warriors have a bunch of proc type things which makes them hard to play boxed optimally too.

However, the DK brings you the 13% spell power buff to a caster group, or significant melee buffs like 20% haste from deep frost. A Pally gets you an aura and a blessing, which is decent but from the standpoint of increasing the power of the rest of the group, the DK is probably peerless.