I'm leveling my DK's for Cata as a possible replacement for my DK+Rets team. My thoughts in logical order:

1. Unless things change drastically, no melee class can *effectively* heal while melee'ing. Paladins can Word of Glory but you're giving up a lot of damage. Shaman require 5 stacks of Maelstrom Weapon so it takes awhile to get there. If you can't heal, then you need defensive cooldowns/passives...

2. Rets lost a lot of defensive abilities with Cata whereas DK's maintained/gained defensive cooldowns. None of the other melee classes is even close to either one defensively.

3. DK's gained a lot of burst damage and made gains on already-impressive AE damage via diseases spread with Pestilence.

So basically it comes down to whether or not Paladins can effectively melee while HoG'ing their team. Once 4.0 goes live we'll have 2-4 weeks to try it out and see if mechanically it'll function. If it does, I'm going ahead with the DK+Rets. If it sucks I'm going with the DK's.

As an aside... Necrotic Strike is going to be insane boxing. Stopping ALL healing on a target is going to just be... Insane. Insane.