Studies have shown that off-topic communication can make online communication more informal and less threatening. Basically, it can help people remember that the other people are humans, not list-bots.
Also, this entire thread of conversation about what should be talked about on the list is off-topic with regards to the topic of whether or not it is in a banned user's best interest to attempt to appeal a ban. Not that I really mind, just pointing out the irony....
Alternatively, one could say that nothing is off-topic, since everything is related to everything.
Prohibiting off-topic communication is also discriminatory against the average woman. Men, as a tendency, are often more to-the-point than women. Indirectness may be considered off-topic by some. (It is also known to annoy some men... sorry. ;_;) Women also tend to express their feelings more, which may also be considered off-topic by some, which in turn may be viewed by the woman as insensitive and perhaps emotionally abusive, making her feel unloved. Hence, prohibiting off-topic communication may further the male-dominated aura of the list. Not that there's anything wrong with being a man, or breaking stereotypes (I really like women who break stereo-types), but a woman should be allowed to be feminine! Men should be allowed to be feminine too... in fact, I would be much happier if more men would be emotional with me.
Historical discussion may also be considered off- topic by some. But do not forget George Santayana's warning, 'Those who do not remember history are doomed to repeat it.' Of course, there is also Charles Sheffield's corollary, 'Those who remember the past too well will never do anything new.'