I wanted to post, i totally disagree....
shadow priest in TBC are one of the most demanded class for Raid content. The top guilds, Dnt, Nihilium, you name it, run with 2-3 shadow priest.
They usually end up in the top 10 of the damage meters (because they dont AOE) if no AOR you can be sure they are in the top 5.
However that is not the utility of the priest. the priest regens health and mana, mana being the important one. While doing this they start to generate much more threat. 0.5 for each effective healing done and 0.25 (i think) for each effective mana regened.
but that is the bonus of the shadow priest, mages cannot dps without one, a mages goes OOM in about 5 min, which is a problem if the fight lasts longer. if grouped with healers, they basicly never run out of mana and so on.
Saying Shadow priest are not good in Raid is just pure bullshit.
As for Arena but i'm sorry that's wrong again. Shadow priest are the kings of 2v2, and extremly powerfull in 3v3 but they kind of are worthless in the most important braket the 5v5. holy the best pvp for this. (crit immunity anyone?). If you dont believe me, go and have a look at the top arena teams, look at how many run a shadow priest and how many run holy priest.