Many argue that BIM requires all work be done in a BIM authoring tool, like Revit. BIM does not exclude the incorporation of legacy data that exists world wide. In fact, for BIM to evolve the AEC industry it has to accomodate decades of hand drawn and CAD produced documentation. Does this information have the ability to contain and share the same volume of information and capability that a Revit model does? Absolutely not, however, legacy data provides valuable historical and as-built records that are useful for informing the design team while they create and/or re-create all or part of a project in Revit.
I suggest that there is far more renovation, remodel and addition work and less new construction. There is far more built environment than under construction. BIM is well positioned for new design, construction and operations where as much of interior design works in previously occupied space. Most of that space was not designed using BIM in its modern context. It only makes sense that to use Revit for commerical interior design, a designer must consider leveraging existing electronic files as part of the design or to build on top of. Until BIM is fully adopted in the AEC industry and all existing space and architecture is recreated using BIM authoring tools, it is logical to use CAD files as part of a BIM workflow.
Most fees do not have the capacity to re-model an entire facility or space for a renovation project. When the technology advances to streamline the 3D laser scanning to Revit model workflow and reduce the cost, then designers will opt for this path. So, for the near future, leverage scans of hand drawn drawings or CAD files in your Revit project to leverage the best of both worlds and available legacy information.