Figured I'd shift the discussion out of the stickied thread for new/returning players, so New Thread(tm)!
Let me start with a caveat that I haven't really been near the raid game for like a decade or so, so this is all more theory than anything, but Sippen asked why a SK would be necessary for a raid. I don't have a great answer, since I also have never played a SK (though I know a few really kick *** ones). I also think that's somewhat of a narrow question/requirement. Very few classes could be called "necessary" for a raid. You could probably take any given raid and max out your capabilities to that raid by including just the exact numbers of each class needed for that particular raid and allow only that exact count of those exact classes on the raid. I'm pretty sure no one does this though. You start with what you have and you make it work. Certainly, raid guilds are going to recruit with an eye towards what they need, but that becomes a broader topic.
The question here is whether SKs fill a need for a raid that no one else can do better. I can only really think of one: They make arguably the best trash tank in the game. Warriors are great for taking hits from the big baddies, but have no self healing capabilities, and relatively set damage. So when you want someone to take out the main target of the raid, with tons of healing/debuff/whatever support, a warrior is king of the job. Knights are generally better at offtanking and trash tanking though. They require less healing (against anything that doesn't require *tons* of healing, obviously). However, out of the two knights, paladins are clearly better at straight up offtanking duties. They have similar mitigation abilities to SKs, but much much better self healing capabilities. And, of course, they have other healing/curing abilities that benefit the raid itself. While this was long ago, I do recall a raid where basically I was offtanking one of the main baddies by myself, with one druid assigned to "spot heal, as needed". A SK could probably not get away with that. The flip side though, is that SKs have much better damage than paladins. So if you just need someone to stand there and occupy a major mobs time without dying, a paladin is best. But if you want someone who can take a more medium level threat on, with minimal healing needed, and actually take a bigger chunk out of said mobs HPs along the way (ie: trash tanking), I'd think a SK would be the superior tank for that job.
SKs are kings of swarming, so they'd logically also be kings of trash tanking. A half dozen mid toughness mobs pop and for some reason must be engaged (can't be mezed maybe?), a SK would be the best at rounding them up and managing them (pretty sure SKs have better AE agro than paladins at least, and probably warriors too). Whether this qualifies as being necessary on a raid? No clue. Obviously, it would depend on the raid. A few months ago, I got picked up for an anniversary raid (no clue the name, but it was in an alternate version of ToFS, which was cool). The final fight basically just required a single tank fighting the main bad, with the dps stacked on it, and support folks standing at a distance healing/supporting. Pretty regularly, different types of mobs would pop up and start whacking people. As a paladin, I could grab one or two and try (feebly because my AE agro ability appears to be broke) to get more, but it was difficult to do the one thing I was assigned to do. A SK was much much better at it. And yeah, I'm sure that fewer people would have died if we'd had more SKs on that particular raid. So there's one example that I can think of, just out of my pretty limited experience.
I agree that it would be great to have someone who plays a high level raid SK to come in and discuss this. I'm sure that player would have much more to say about this than just us theorizing about it. I agree that SKs are pretty low on the recruitment lists. But I think that's not because they are absolutely unneeded, but that the ratio of SKs to need is a bit warped due to the high soloability of SKs as a class. So pretty much any raid guild is going to have more SKs than they will actually need. But that's not to say that the SKs they have don't fill useful roles. Again though, this is mostly just theory on my part.