You can easily spec one of them to be Blood as well, if you want a dedicated tanking toon.
That one then does +threat moves, while Frost does great DPS...
Howling Blast is 30 yard range, hits everything within a 10 yard radius of the target, applies a Frost Disease which snares the targets, and you have a talent (in Unholy) to instantly restore your Frost Rune once the disease is cured.

Some people have cleared most of the heroics, in the earlier stages of the expansion when we outgeared nothing, by running 4x Blood plus the healer.
This was done in PvP gear, where Resilience eats a large portion of the iLV stat budget and does virtually nothing for defenses against mobs.

Blood x4 would not be fast kills, but it is doable.



It is similar to an idea that was popular near the start of Cataclysm, on the boxing sites.
Running with two dedicated healers, and then three of whatever class.

Two Paladins for example can gain self heals, whenever they heal someone else.
And also set their Beacon of Light to heal another target for a portion of the amount healed.
If they were to both set their Beacon to the tank, they could AoE heal the group by healing the other two DPS.
Or if they each had a DPS as their Beacon target, the tank could get alternating heals (to preserve mana) which would certainly top of the DPS (Beacon of Light targets) for incidental AoE attacks.

Either methodology gets you reduced DPS for increased survivability.



I'd also consider a team specific to PvE.
And another specific to PvP.

You could have something that is near optimal in both aspects of the game.

I do like the idea of one team, that can run both aspects ideally dominant in one and passable in the other.
That way you can collect achievements on the team, from every aspect of play.

Go with whatever is fun.




With the whole LFR (Looking For Raid) system, PvE gear is rather easy to get if you want it.