Exchange 2007 and 2010 were architect with certain technology constraint that existed at that time, where CPU performance was the Key constraint when Exchange 2007 was released and to alleviate the SITUATION Server roles were introduced. However server roles in Exchange 2007 and Exchange 2010 are tightly coupled

Nowadays, CPU HORSE power is less expensive and it is not a constrain factor, with that constraint lifted, primary goal for Exchange 2013 is simplicity of scale, HARDWARE utilization and failure isolation. So Microsoft reduced the number of server roles to TWO as Client Access Server ROLE and Mailbox Server Role

Exchange 2007 and 2010 were architect with certain technology constraint that existed at that time, where CPU performance was the Key constraint when Exchange 2007 was released and to alleviate the situation Server roles were introduced. However server roles in Exchange 2007 and Exchange 2010 are tightly coupled

Nowadays, CPU horse power is less expensive and it is not a constrain factor, with that constraint lifted, primary goal for Exchange 2013 is simplicity of scale, hardware utilization and failure isolation. So Microsoft reduced the number of server roles to two as Client Access Server Role and Mailbox Server Role