Common Data Service and Dynamics 365

Common Data Service
The Common Data Service provides the known base schema that can be expanded with custom entities.  That means that regardless whether it's organization A or organization B, you can be confident that the same base entities will be in both those schemas, allowing not only internal applications be built, but as we look beyond that, where we build applications that work with the Common Data Service, we have a known set of data model that will always be there. Data doesn't have to be synchronized with the Common Data Service for all scenarios.  Many can still just use the connector.  So you have to evaluate each scenario to see what best fits the needs of the application.  The Common Data Service in the context of Dynamics 365 can be very helpful to support data storage of purpose-built applications that wouldn't need the data stored in Dynamics 365.  But that is something you'll have to consider as you build applications: Should the data be stored with other data in Dynamics 365 -- meaning it would be useful and is more enterprise-related data -- or is it something that's more tracking, or transitory data or data that would be better suited in the Common Data Service.  Both of those are options, and you'll have to evaluate them on each individual application case.