Imagine you're running a consulting company. A client approaches you with a project.
Like all software projects, there are big error bars on all time estimates; it could take X hours or 0.5X hours or 2X hours, even without scope changes. With scope changes, all bets are off. The deal you strike with the client makes this clear - you estimate X hours, but you can bill them anything from 0.5X hours to 2X hours.
If you assign one competent engineer to the project, they'll bill X hours.
If you assign two competent engineers to the project, they'll each bill 0.55X hours for a total of 1.1X hours billed (the extra 0.1X is due to communications overhead)
But if you assign one competent engineer and one incompetent engineer, the competent engineer will bill 0.8X hours completing the project and the incompetent engineer will bill 0.8X hours - for a total of 1.6X hours billed.
In other words, if a project has one competent engineer, adding a competent engineer adds 0.1X to revenue but adding an incompetent engineer adds 0.6X to revenue.
How do you suppose these incentives work out in practice?