Only for ToGC 25 Anub/Halion OT.
Other than that, you will be fighting against "better" tanks such as protadins, BDK's and bears. Often replaced by one.
Combat rogues are everywhere.
Disc priests are common as well, but sometimes there's need for them, just because X,Y,Zs are already logged their A,B,C's instead of D-isciplines.
Enhance shamans aren't wanted either. They can dish dmg but other than that, there are better options.
Demonology Warlocks, Balance Druids, Shadow Priests to degree are wanted.
Even still.. Playing Demonology Warlock because it's on demand isn't fun. You'll level to 80 and see dozens of (Demonology) Warlocks all of a sudden filling your screen.
Play a class/spec combination you want, though note that some specs aren't desirable or "perfect" choices.