Both sides have a point, because the impact of local events can be important but not always drastic, it depends a lot.
It is true that for some trips, like the Grand Magal of Touba, accommodations and transportation are heavily impacted, but that's very specific.
For a business trip to Dakar, for example, a distant regional event will have no impact on costs or availability.
It is necessary to evaluate case by case, considering the specific destination and the needs of the traveler to find a middle ground.