"I'm sure you've split threads..." All the time. Actually, I don't let them get off-topic in the first place. I'm ruthless

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It can be tough (note, I'm trying to stay on the "off-topic" topic), since on one hand casual conversation between members does help build the community feel. On the other hand, it makes it very frustrating for new members, or potential new members who find a topic through a web search. They end up wading through pages of humor they don't understand from people they don't know, when all they really want is information the thread is supposed to contain. Sloppy and unprofessional.
My philosophy has always been to keep threads high-value and on-topic, and to keep the humor and casual conversation in a "lounge" area.