Sure SIP would allow simple interop, but it would introduce a third user experience (e.g. how to forward a call or set up a conference call).
Remember SIP is more like SNMP than like WiFi. It's very open ended and allows for all sorts of implementation specific details, and it generally doesn't define the user experience.
While SIP will play a role, Aura is not the savior to compete against Cisco. Product rationalization is going to be hard.
O yes, and the world has moved well beyond VoIP to UC.