If I start answering someone's question in a large group, but then see someone else starting to type while I'm typing and I know that that person knows a lot more about the topic than I do it means I will stop typing. This helps reduce the number of people responding with the same answer to one question which results in a different kind of "busy" distraction.

I'd like to add that I think of a scenario where some forums are readable by everyone in the organisation, but only certain members are allowed to post. This will allow teams to keep discussions relevant, while the global search will let others find any content that might be relevant to them. If they then wish to ask questions about previous posts or messages in a moderated forum, they can either contact users directly or send a message to the forum that gets sent to forum moderators who can then decide if it is best to handle it off-forum or post it on their behalf.

The idea would be to keep forums topic specific, not to hide information or remove spam (there really shouldn't be spam anyway).