I think tattoos may have been used to see who is part of your "gang" or"group".
just an idea, not completely sure.
thatd be an interesting idea, but i think its just a coincidence.
they could have had them do extra small things like add even more effects, like Planescape:Torment did, but its already been released and i dont think they want to mess with existing things. the idea of having them show clans/guilds would only really work if the tattoos were customizable, and even then, its only like 20 pixels and really limited. im sure theyll come with some interesting ideas for guild icons (flags, sig badges, etc,i cant think of any others lol) that will be even cooler. ^_^