One of the downsides with moving data off the platform in an archive use case or with not being able to replicate data onto the platform is that the end users have to move between applications and logins to view data; this causes an overhead as the process and data is not connected.
A relatively new feature of the platform is the ability to surface external data within the Salesforce user interface via the so-called External Objects and External Data Sources configurations under Setup. They offer a similar functionality to Custom Objects, such as List views, Layouts, Custom Buttons, Reports, and Dashboards. At the time of writing, Salesforce supports Microsoft Sharepoint's external data sources in this way (through a beta program), though it is likely that this will be extended in the up coming releases. This technology also offers some interesting integration possibilities for the future.