I just recently joined the thieves guild. One thing I've noticed is how Mavin Black-Brier and Mercer Frey love to throw around their connections to the Dark Brotherhood. One problem: I'm the Listener, and I have never met them in my life. Just once I wish I could tell Mavin to shut her mouth because I am the Dark Brotherhood. Anybody else ever feel that way? Would it have caused too many problems, programming wise, to synchronize guild affiliations so as to make the games' villains not lot so stupid?

I RPed my own answer to this issue. Killed the dark brotherhood. Now Maven can threaten all they want and well. no one is listening.

But yes. its an oversight only corrected slightly by using Delvin during the Dark Brotherhood quests.