New to multiboxing

[Malochos]

now what do u mean by some heroics? wouldnt 4 pallies and a DK destroy any heroic?
and how about 5 pallies for pvp or is the DK essential becaue of grip? how about 2 DK's for more grips?

[Ualaa]

Four Rets and a DK have done all heroic 5-man content.
And usually sooner (in worse average gear) then predominantly caster based teams.
You gain plate across the board, instead of mail + shields.
But then everyone is in melee, faces cleave and/or splash damage etc.
It is a trade off.

The team is amazing in PvP.
You're in their face immediately, so they have no reaction time.

If you look at essential buffs, from the pallies.
Devotion Aura, Retribution Aura, a Resistance Aura.
Blessing of Might, Wisdom, Kings/Sanctuary.
Judgements of Light, Wisdom and Justice.
Three pallies is decent, a fourth doesn't add a whole lot, from a buff point of view, but an extra DS/DS and additional passive Divine Storm heals is not inconsequential.

A 2nd DK gets you an extra Death Grip. But that is pretty much it, as the second DK isn't bringing anything to the team, that the team does not already have.
And the DK has substantially lower DPS then a Ret.

I could see going with a Warrior for both Mortal Strike (50% now, 20% in Cataclysm) and the ability to break immunity effects such as the Pally Bubble.

An Enhancement Shammy would similarly add a lot to the team. Totems provide root and root protection, some spell protection or resistances, health regeneration or disease/poison cleansing. And of course the big one Heroism/Bloodlust every 10 minutes.

The fourth Ret, is quite a bit more DPS then a boxed Warrior, Shaman or DK. And provides passive benefits or an extra step on round-robin effects.

The first DK adds a lot to the team, more so then a fifth Pally would. It is arguable, who adds more... the DK or an Enh. Shaman, but the DK also open all heroics which is PvE progression too.