It will be prudent to include buffers in the project plan to account to the unexpected. This is a practice in two companies I have worked for developing medical devices and combination products. I always include buffers in my timeline and DDP. Your example of marketing department going off and doing their own thing and impacting the project team can happen in dysfunctional organizations. However there are many other aspects during development of a product that can bring change to the project plan such as change to the actual product (e.g. design changes to improve performance). The important point is to understand that your gantt is a living document just as other documents (e.g. DID) that will change at least once to several times during the project development.