You have to decide when it is better to use Independent Professionals as consultants or contractors rather than struggle to hire full-time employees. It doesn't make sense to pull people off regular operational jobs for projects. If their work was important they have to be replaced. If it wasn't important they shouldn't have been doing it in the first place. If their reassignment simply means more work for others, there is a price to pay in burnout and degradation of morale.
Employees lack required skill sets.
Sometimes work comes up that regular employees are simply not equipped to handle, e.g. process redesign, organizational development, long-range planning, unique procurement, problem solving. Assigning people work they don't know how to do rarely produces satisfactory results. Training for one-time new tasks is not cost effective. Learning curve expense is often overlooked. Employees are overloaded, burned out Employee abuse is not good business.