I think this is something that they have never really considered owing to how different specs were really dependent on different gear in the past. Now that we are well into the era where your armor has different stats and even set bonuses based on what spec you are currently in rather than what spec you chose when you turned in your token, it might be worthwhile investigating the possibility of having spec-dependent enchants and gems. Raises issues though because what do you do about the person who doesn't want or need for their enchants / gems to switch when they change specs?