I imagine there are many ways that making guild tabards matter a bit more. I for one would like the gold used to pay for them go to the guilds bank rather than going into the void! Me and another person had a joke saying "I donated my gold to the Void, and all I got was this lousy shirt!"
Even something small like that, though not much, is something. There could be other things like a small boost to xp or something (like, quite small) when grouped or questing with fellow guild mates wearing the same tabard (much like the ring of mara) I find that most guilds, even when doing group things like fishing, don't don their guild tabards together, maybe just a couple, it would be nice to give a little incentive to do so.
"You are brave and you are strong, of this there is no doubt. But without me at your side, your deeds will go unknown, your death unwept and your story unsung - Sings-In-Shade, the Argonian Bard