But in all honesty, are these addons really necessary for raiding? There's some interesting arguments both ways.
One of the most convincing that I've heard is that if Blizzard really wanted you to use things like Omen, they would have built a threat meter into the game itself. That does make sense in a lot of ways. The rebuttal to this argument is that Blizzard provided a huge Addon API for programmers to make things like Omen.