Imagine a raid, full of Enhancement Shamans, Blood DKs, Survival and BM Huntera, Frost Mages, Arms Warriors, sub Rogues, Disc DPS Priests, Prot Warriors (LuL, had to put em here), and many more fun specs. :D
All im saying is, there are 10 classes that have atleast 1 viable spec, there has to be a high chance one of them will make ur gameplay fun.
Sure, Blood can be fun and all, but dont expect to be inved in the raid if u play a "fun spec", especialy if pugging, since there, every DPS point is important most of the times...
The main reason some classes are needed in raids are buffs and debuffs they provide.
Once u get every single one of them, u look for classes that deal most amount of dmg overall.
BiS Fury Warrior = 2x BiS Blood DKs (not even jking).
So yea, play whatever u want, just dont qq if people dont want ur fun spec in their grp. :D