I would also like to know the purpose of the application server role. You say in the article 'Since we will be installing SQL Server 2008 later on, we will have to add the Application Server role on both of the nodes' that makes it sound as if it's required.
Yet this technet article http://technet.microsoft.com/en-us/library/cc754024(WS.10).aspx states that 'Not every server application benefits from the installation of the Application Server role. For example, the Application Server role is not necessary to support Microsoft Exchange Server or Microsoft SQL Server on Windows Server 2008'
Could you please clarify. Is the Applicartion Server Role required or not?