Ah my bias to warlocks has revealed itself (it was my first level 70).

I wouldn't say a lock is a must have for kara. There are certain fights he is great for and would make it easier for a multiboxer.

Terrestrian illhoof - sit him fairly close in to the portals and have him spamming Seed of corruption on illhoof. Especially if he has that pally buff for non interrupt.

Curator of course - an amplified curse of doom timed to go off in the evocate phase.

Aran - the healthstones are great after he sheeps everyone and does that big blast. I guess that would be the best use of healthstones, and the easiest to multibox up (bind the same key and all your chars eat their healthstone at the right time). Banish one, chain fear another on the elementals that pop.

Opera event - The Oz one. Takes roar out of the equation with chain fearing. (of course you don't know which event you are going to get).

In retrospect I think the utility the warlock brings is great, but would take too much micro management in a 10 box raid.

The more I think about it though - the more fights require a lot of movement and differing talents of the various classes all needing to be done at the same time. What a challenge...