If the Ret is faster, it might not matter over a short distance.
If it does, lead with another toon even if you switch back to the Ret after you get there.

What does a Ret bring to the table, as opposed to another DK?
The DK's aren't going to lose any abilities, given you already have three others.
Just uses of abilities...
The big ones seem to be Death Grip, Strangulate and Hungering Cold (if you use that).
Also I suppose the Mind Freeze every 2.5 seconds would be Mind Freeze every 3.33 seconds (on the kill target).

DK's are quite a bit stronger than my Feral team, entirely because of their aoe/cleave type attack.
Howling Blast is usable at 30 yards, hits everyone within 10 yards of that target, applies a 50% snare which does damage over time and doesn't break Hungering Cold...
The Howling Blast is incredibly powerful, and this is the main thing that you're going to be losing...

If your healer is a Paladin, the Ret brings one extra buff (Might & Kings, instead of one of them).
Kings is pretty huge, if your healer (5th slot) is not a Druid or Paladin.





I don't arena often...
But running into a group...
Popping cooldowns:
- Summon Ghoul Pet
- Anti-Magic Shell
- Icebound Fortitude
- Lichborne
And going crazy on one target, with numerous Death Grips...
Pop Empower Rune Weapon (for all Four more Hungering Colds)...

In my successful games, usually results in 2-3 opponents dead in the first five to ten seconds.
Anytime they're bunched up, they seem to lose.

If the games go longer than a minute, I generally don't win them.