This is one of those things we'll have a better idea about after wotlk goes live. I have tested with a pair of shamans and my pally but I don't have 5 beta accounts so I'm not 100% sure which I like better. 4-5 shamans with Thunderstorm on round robin is definitely insane but giving up the insta-gib is hard.

I don't think the insta-gib will be as necessary after hitting 80. With hex and thunderstorm a shaman group will be more able to control the pace of a fight.