Eh? I'd think there would be more cases where a raid might be short on sheer numbers of tanks, and thus want/need a SK, versus not having dps/debuff/whatever that might make them want/need a necro. I'm sure some will disagree, of course, but IMO, while SKs might not be a sought after class, I'd think they'd have a better shot getting a slot on a raid than a Necro. Everything else being the same.
I've had enough raids, maybe not fail, but be inconvenienced and/or delayed because "gosh, we could have used just one more tank able to take trash hits along the way" to make me think that a decent SK will usually be welcome on a raid.