History: Tiki Suite Name vs Number

When setting up the phone system, do we keep extensions? or do we just do everything with firstname.lastname?

Why use phone numbers (voice or SMS) when you can contact people via XMPP?

Is it important to do SIP + XMPP or should we just do XMPP?

"In simple terms, the phone number remains the main unit of currency in the majority of open source PBX installations. In the world of unified communications, the email address is fast replacing the phone number."
Source: http://www.opentelecoms.org/asterisk-or-freeswitch