This is one of the ways of remaining a little less censored. Local rules seem to be put in place by the majority of users to censor someone. However, its not the full blown self moderated way of censoring. At least its out in the public. I feel like local rules could definitely be applicable in auction threads, but elsewhere I probably wouldn't use them personally.
I think local rules would find their place in threads which require additional format/structure to the replies. This will streamline things like the "Known Alts" thread or the "Signature Campaign" Service Discussion thread.
Offering the option of drifting away from any asides and (not-exactly-on-topic-but-borderline) semi-off-topic replies is a good thing, in my opinion. Gives a bit of control to people who don't want to see pedantic or vague replies on their threads - to people who want to actually have a genuine discussion.