I think that it's simply the decision that makes the most sense for Blizzard, because of all the reasons you all have mentioned already.

The LGBT community is happy because they got a 'high-profile' lesbian character.
The players don't need to care because literally nothing has changed in-game because of this (though admittedly the idea of her having some sorta wobbling when aiming at female characters is pretty funny).
The worst rational reaction people can have to this is exactly what you are expressing: questioning the necessity, effect and so on of the decision. And there's nothing wrong with healthy debate.
The only ones who react truly negatively are people the majority shrugs off anyway.

So yeah, I think there's no agenda (gay or otherwise) being pushed with this. It's just an advantageous move for Blizzard's image and after all, what else do they need to care for?

Edit: Also [@BrokenPromise]. Much love. :P