I also think that guild tabards used to be way more important significant in the past expansions. You used to be so proud being able to wear a particular guild's tabard - especially if it took you ages to get accepted.
I remember when I applied for the raiding guild and had to wait a week... I was so anxious and afraid that they would deny my application but luckily, I got in. Right away, I put on their guild tabard to show it off!
Nowadays though, the guild tabard doesn't really hold much value. There are so many other, better-looking ones that fit your transmog or your chosen look.