Productivity per hour drops, but it doesn't matter to the agency because all of their employees are salaried. It is not people sitting in a room not being productive per se, but it is a bunch of burned out employees not being as efficient as they could be.

Again, this leads to burnout and employees that are sufficiently burned out either quit or are fired for no longer being able to meet the productivity requirement and/or making mistakes. The agency does not care because the supply of recent college graduates in marketing and communications is never ending (it is one of the most popular majors at almost any college/university).

Interestingly, slower work to a point is better for the agency since they bill the client per hour. That reverses when the quality of work suffers or the employee is no longer able to get everything done that was promised to the client. The agency gets more value out of a single employee when they work longer hours. Bonuses are common in the agency world, but they are typically meager compared to the extra hours worked. Burning out their junior employees is then to the advantage of an agency that bills by the hour.