So if an employee produces 0 or negative (as happens in several organizations) but receives wage X, are they exploiting their employer?

Microeconomic models that assume the marginal productivity of a worker can be calculated in advance are drastically at odds with the reality I see, where employers are terrified of a bad hire and even if they get a good hire, they may still end up with zero to show for it because of bad management or simply market conditions.

You could argue that the reason we have corporations at all is to spread the risk of unknowables across many people, so that it becomes bearable and not everyone is exposed to ruin if luck doesn't turn their way.