IM in the Enterprise: Why?

Phil writes about IM in the enterprise.

I've not been able to determine the advantage of IM over email in the enterprise. More than 9 times out of ten, email performs as quickly as IM, and more than 9 times out of ten the message I want to send can be treated asynchrnously anyway.

What's the critical scenario for the IM argument over email? The conversation is taking place at Phil's blog.