Im not sure if there was ever an official answer about this, but this user used to be an admin. Maybe theymos has the same opinion about the matter.
I wasn't saying there's never a valid reason to know when someone is banned, there are some. OTOH it could also be abused. For example, if you know someone is banned, then you could make a new account and impersonate them, or attempt to get them punished for ban evasion.
I'm inclined to think it's not really anyone else's business. If you were to get banned for posting granny porn would you want everyone to know that? People can usually still change their signatures and personal message (below the avatar area) if they want people to know they were banned, and why (though the why would probably be misleading).
And there have been an old discussion about this here:
https://bitcointalk.org/index.php?topic=1674903.0Also, IIRC Vod was developing an extension or similar that shows some info about the user directly on their profile (things like if he is banned).
(I may be wrong though)