I'm not convinced that chat is currently working as an alternative to long discussions held in comments.
One problem with chat is that there's a widespread perception that chat systems are intended for conversations with all participants online at the same time. Even in the chat FAQ, it is not clarified that chat messages intended for a particular person (addressed using the @ syntax) appear in that persons inbox. Further, it appears that there are some significant practical issues - according to the FAQ if the person you want to discuss something with has never used chat before, or never used the chat room most appropriate to the discussion, that message won't appear in their inbox, so at the very least you'd need a comment telling them to go the the chat room.
Another possible issue is more psychological. A very common use of comments is to inform the author of a question/answer of an error, or suggest an improvement. This isn't "seeking clarification", but it seems to be normal practice. However, once publicly "accused" of making a mistake, it's only natural that people will want to defend themselves in the same public place.
So - can chat really work for this? Does it work as well as it needs to already? Or does it just need a FAQ update, or some other marketing to improve awareness? Or is some alternative method for handling comments needed?