Mcing for Aura of Celerity is a nice thing, notice: The aura effects your group only and not the entire raid (unles blizzard will fix this reset that it will become raid wide).
When you MC you will have abilities: one of them is the aura, once the aura is used your group will be effected by it - no need to spam cast it. Notice: if you MC and he already had the aura you need to refresh the aura so it will effect your group.
The MC mob has additional melee hits and a strong melee ability, meaning you can DPS while you MC (when i MC the mob melee hit for 8k-10k and his special attack did 20k-25k was pretty cool).
When the Dark Rune spawns in the pit, it is abit problematic to keep the MC up, the situation you will have is MC breaking every 10-15 secs from damage, untill eventually the mob becomes immune to the MC. The solution is simpley to CC 1 or 2 mobs and MC them at phase 2 (sheeping, hexing, fearing, traping works in 10m, in 25m sheeping i guess will be the best).
In phase 2, when thorim is in the pit and you chose to MC make sure you stand in a safe position not linking chain lightening (damage might break the MC). Make sure people who can stunn/taunt/slow the mob will be aware of your location and aware that this mob can kill you for Mcing him a hammer of justice can allow you to re-MC safely. In the 10m i had to re-MC once in phase 2. i have yet to do this in 25m (although did MC several times) but MC doesn't go hand in hand with thorim hard mode 25m (though in 10m hard mode you can MC if you aren't a healer, or if your the 3rd healer).
The mob itself can do alot of dmg. But while this time, this is difficult to see what appen to you. And you will lost your mc often. You will lost alot of time to keep a warbringer alive at the risk of your own life.
So we changed our strat. Now i have to mass dispell mobs the whole fight to remove Hots and shields.
It make the encounter more easy as the adds will die faster. So it worth the lost of dps.