My personal opinion is the Alliance may come out looking stronger after siege without taking advantage of their vulnerability. They may become respected more for their mercy or their mercilessness, an act of kindness that Horde may not soon forget and in time withdraw from Alliance territories out of respect for King Varian.
I think the real reason though is that in Cataclysm we felt the ramifications of updating past content for the sake of lore or faction developments. It is detrimental to the time and resources spent on current and future content. I'd rather just have it stated lore-wise that Alliance are gaining some ground over the horde than see it in game if it means we can get more content quicker and at a higher quality.
(Gameplay needs to take a priority over storytelling in this case.)