Shouldn't take too long? Huh? You just have one extra dps.
Normal kara runs are 2 healers, 1 tank, 7 dps. Now you got 8 dps! Omg! That's like 10% more damage, woohoo!
One fuckup from the shammy and you're all screwed.
Plus stuff like
Aran, Nightbane, Illhoof, is gonna give you some trouble, especially since you gotta heal many targets at once.
Easy for a resto druid since he can just hot everyone, but a shaman doesn't get that convenience.
An unlucky chains during Illhoof and you're all dead
And for the guy who said the SP going in Holy gear - Well, that's no longer 8 dps, is it?