After a couple people calling me a troll, I decided to justify my claims. All tabards, for me at least, tell me that they give a boost in stats. But the stats end up not changing.
(Note: This Boost in stats is for every tabard and not just the Guild Tabard)

I'm pretty sure that this is just a visual glitch, but it's kinda odd anyways. I just wanted to post some proof to show that I'm not getting senile or something.