Titans to some degree also can be considered 'optional'. I'm referring to Hard here, of course, not normal. Warlocks can self-resurrect, which in Hard is almost a necessity, because mistakes happen. A 2/4 Titan/Warlock raid is almost ideal, because two shields is incredibly useful, but even a raid of all Warlocks would be a walk in the park because mistakes don't mean a raid wipe.