The tendency is to assign the difficult tasks and increase the workload of the most productive people in the company. While it may work in the short term, those that are stretched too thin are unable to sustain excellence, over time, in all facets of their work.
Stressing systems, and individuals, is never a good thing. Like seasoned marathon runners, the best must pace themselves, or they will run out of steam. Besides balance makes the journey better for everyone involved