It's absolutely a major lore development for them, but development of the lore in general was not soley (or rather, primarily) focused on them for that addition to the game (I.E., the Cata 1-60 revamp). That's why I call it non-exclusive.
The lore hasn't had a FOCUS on the Forsaken since Wrath. Forsaken lore hasn't been EXPANDED since Cata. See the difference?
Well, this is exactly the problem with 5.3, isn't it? Alliance players don't want to follow Vol'jin for the Siege of Orgrimmar. They want their own side to the story.Pretty sure the Alliance is helping Vol'jin because that's who they're going to have to follow for the seige of Orgrimmar. If this is the case, then why would they want to water that down with Alliance involvement elsewhere? It detracts from that focus, I'd say, or calls that focus into question, which isn't necessarily a BAD thing but it does ruin the immersion.
As you say, there has to be one "true version" of the raid. No matter how it goes down, it will always be frustrating for one faction. Either the Alliance follows the Horde in and they (rightly) feel left out, or the Horde follows the Alliance in and they get left out. I think the whole plotline was short-sighted of Blizzard.
For the Alliance, killing Garrosh was supposed to be about revenge on the Horde against all they've done. Now instead, it's actually HELPING the Horde and making them stronger.
But I don't think that a faction's going to feel left out here. The player group is probably the special task force of the invasion, and the Alliance and rebel Horde are working together, it's not one side leading the other, unless you really want to say the Alliance is following the Horde because the Horde members there actually know the place better, which is logical... but it's a partnership. The Alliance is providing a lot of the strength and the rebel Horde is providing the intel.
Any way you slice it, the Alliance is going to be the one on top after the Seige of Orgrimmar. The Horde as a whole will get a moral victory, but it certainly won't get a tactical one.
Actually having a sensible leader would greatly strenghten the horde.
Alliance can just sit back and watch as Garrosh's fails bring everyone, including the pandaren along with the shado-pan and probably the Argent Crusade, on their side. They don't do it jus cause WoW cannot exist if one faction just defeats the other.
The Horde'll get a moral victory and it's long-term survival will be more secure, but it is by no means strengthened tactically, strategically, or numerically.
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