You should basically exclude a source from media planning if:
You don't want to include the source in the recommendation.
It's a source where the spend can't be changed due to internal restrictions.
It's a source where you need to spend a certain amount as it's important for your branding and performance is secondary. A good example is TV or adwords brand.
By the way:
By excluding a source in the media plan we only consider the past spend for the calculation of the media plan for all other sources.