This doesn't invalidate people's requests for citation or supporting arguments. By the current logic we should simply listen to everyone because they could be anyone. This is quite obviously, self defeating.
That being said:
The expertise stacking was by Xav during their Ascendant Council HM progression. Tank damage is largely trivial and the number of sub 1% wipes that under-geared guilds were bouncing through was getting pretty atrocious.
As always: look at your current progression encounter and gem/reforge/flask/spec accordingly. People who are looking for three golden rules to successful tanking need find only one: "Think"