I'm a DC fan for large party tanking. Shield defense bonus (which for you is 360°) combined with +4% heals means overall you take less damage in DC than you do in NM. You require almost 7% less healing than a NM set, which is more than the difference between Doom and NM in pdef. The reflect damage on NM is such a low % and you have nothing special to stack with it. 10 extra damage is simply insignificant...the idea is to kill the mobs quick so they don't attack much anyways.
DC set is also cheaper on my server, though to be honest that really wouldnt stop me; I own several A sets. Back during ABG AOE, you could see the difference pretty quick. Bring a DA in NM vs a Paladin in DC, and I was tanking the big boys well before the equal level DA, and you could use Vrock to take away the 360° shield block advantage.
Agree with sith on this, the DC heavy set is often overlooked and its not a bad set at all for a big party tank.
A lot of people love their doom and they chose to stick to an OE doom set all the way to imp also.
Im also a big fan of DC for another reason, practically no one wears it making you look unique and it does look awesome on a TK or human for that matter.
Another perk to the DC set: Since sleep got nerfed, the added resistance to sleep that you get with nightmare isn't very usefull. On the other hand, para lands more often now, making the DC set bonus that much nicer.