Yes, that's probably true, both in the sense that you'll always be excluding someone at the margin, and in the sense that people who really don't want to do it, but don't want to admit that to themselves, will come up with some other excuse like this one.

In my case, though, I can take 3-5 weeks of leave from work (depending on how carefully I shepherd my resources) before I have to start negotiating unpaid leave. I imagine there are some other people in the same boat.