You say, "For example a discussion like this could never happen in the CB.", but there's a section for discussing Perl Monks itself. Seekers of Perl Wisdom, Meditations, Tidings, Obfuscation, Poetry, Perl News, and Tutorials are each a section for a different type of discussion, too. Perhaps you just mean that a longer conversation is more difficult to carry on in the CB, and that's true. There's a permanent place for anything that is topical which needs deeper discussion than the CB offers.

Diluting a topic-specific site with permanent completely off-topic discussion threads defeats the very purpose of having a topic-specific site.

The Chatterbox being transient can be a drawback, but it can also be a positive thing. I don't really care to have "Good morning. What's up?" taking up disk space, showing up in searches, and otherwise being a pain in the posterior. For on-topic discussion, there are many sections. Our scratchpads and home nodes can be more permanent than the CB and less permanent than discussion nodes. Your scratchpad and home node don't even need to be topical to PM if you don't want to use them that way. The CB is for chatter that doesn't need to be saved. I'm sure you don't document everything you say during the day offline. Why do so online?