I am going to see if I can make some local rules when I do my thesis defense in a few weeks.
I think that local rules can be useful in keeping a topic on track. I also think that the mods should look at how they're being used in a thread. When threads about major things don't stay on track we see multiple spin-off threads spawned by people who think their particular point has been lost amongst the noise. This leads to useful information being spread across multiple threads in multiple forums and makes it harder for people to keep up to date with what's going on.
I don't think local thread rules should be able to be used to exclude specific posters from a discussion, though. The mods will already remove off-topic posts, and I'd hate to see moderation based on who a poster is rather than the content of a specific post. theymos has given people time out for trolling before and will no doubt do so again. OPs can lock their own threads if the discussion gets derailed and re-open it only when the thread has been cleaned up or split (I do think the mods should use the thread split and thread merge options more often).