I've always been a bit of a grump when it comes to off-topic stuff, thinking that it detracts from the actual content and in some way dilutes the quality, so thanks for a great article proposing a different point of view.
Off-topic discussions definitely aren't a bad thing for a community in my opinion, as long as they're not derailing threads and are done in a separate area of the forum.
Ultimately, an off-topic section, and general community chat threads and such I've always found to be super helpful in building a real sense of community, and building and fostering links and relationships between members. I think this kind of relationship development that we frequently see on forums (think members meeting outside the forum, or teaming up on projects together) would be a lot less frequent without the use of off-topic forums and areas of forums that aren't quite as formal and can serve as a place for people to just chill and get to know each other personality-wise.