In this post I will try to clarify this issue and provide my own definition.
I will come back on these questions in the following, either directly or indirectly.
If there is the need to find a single reason to integrate a non-IMS service with IMS, then the possibility to combine it with person-to-person communication is this one, considering that communication is the core business of telecom operators. However, I hope I have illustrated in past posts that there could be other important motivations for such a combination.
Therefore, the IMS service architecture can go well beyond the support of SIP and access to access to IMS-based enablers. Here is an overview of this scope.
SIP itself and even more the combination of SIP with other protocols can give birth to a flurry of new services, some of them implemented on IMS.