Currently, none of the NPCs in the Merchants' Guild building recognize the full joining of the association. The guy who writes you into the book even gives you the same dialogue options to do it all over again.
It'd be nice to have some sort of physical identifier like the protection papers, too; perhaps some weightless clickie like a pin or badge to throw a [flashes CHARACTERNAME's Merchants' Guild membership thingie] into chat.
Totally support this.
I don't know what the benefits are of joining the Guild but for the money you put in it'd be nice to be able to flaunt it at least
+1