Yeah people will say they are fine and its not necessary, but I beg to differ in terms of what we have now with out playable races. Every playable race in Wow right now, post mists of pandaria, is on a level that is completely different to anything before it.
The reason why the pandaren amazed people wasn't just on how smooth looking their animation and shading was, it was on their expressions when doing anything, ahead of any other race, and when I first played a panda, it felt like I was playing another warcraft game entirely.
prior to them, the nearest to 'updated' from vanilla models were the draenei and blood elves, yet even then the animation style of tbc races were far and head of the vanilla models, we all know that. So when worgen and goblins came out, the animation and character design team were still using the same old technique from vanilla and tbc days of having expressionless models on a painted surface, as opposed to the pandaren whos faces were fulled animated.
I don't expect it soon, but I'm still hoping this will come before the end of WoD if not then into the start of the next expansion. I honestly feel like I'm playing another version of Wow when playing a goblin or worgen, just when compared to every other race now, they need fully detailed facial expressions, smoother skin definition, and smoother animation cycles, before it can feel like they match anywhere near the updated models.
There really does feel like a definitive line drawn between when mists of pandaria came out, and anything before it, in terms of animation and design.