"It is just like email. Make it like outlook". Those words have been said in more than one executive meeting and you and I have both been in discussions where someone ended the argument with "well, that is how it is done in Outlook, so we must do the same to be consistent".
But few make the argument that you should simply not build any UI but just use email. Now, this is different from for instance Dynamics CRM, Dynamics Business Contact Manager, or Dynamics Small Business Application that all build additional UI inside Outlook. Khoi Vinh instead writes about companies that don't make additional UI, but use email as a bridge, an way to exchange and collect information. The idea is to build on something people already use and already has their own workflows for.