For design as such we know that there are some advantages of outsourcing design, such as:
+ breadth of experience,
+ objective opinion,
+ bringing challenging perspective for problem solving, and
+ producing focused solutions.
An in-house design team advantages are:
+ that they are active stakeholders in organization,
+ focused experience,
+ shared sense of purpose,
+ influencing companies design culture, and
+ easier access to decision makers and power holder (Best, 2006).
Both options have disadvantages too. It is pretty clear, although the example goes from using design, that in corporate identity and brand architecture development the optimal solution will always be combination of internal and external resources.
Source: Best, Kathryn. 2006. Design Management: Managing Design Strategy, Process and Implementation. AVA Publishing SA. 215 p.