True. If you can't tell how "hard-working" someone is from an interview, you might end up using "willing to work with less vacation" as a crude filter.
This is probably part of the answer to the whole question. In a world where you could measure every employee's productivity accurately, there's no need to keep track of how much vacation someone takes. In the real world, people end up using "time spent in the office" as a proxy, however weakly that might be correlated with actual productivity.
