The best private conversations start by branching away from a public space. It's that juxtaposition that makes each half all the more special (think noisy club/bar vs outside). I think the biggest failing of IM is that it's easy to end up with a declining circle of people whom you communicate with. Without a public space to replenish ideas from, privacy becomes a curse. IRC allows both.

This is exactly why I loved IRC and continue to use it to this day.