I have seen these used a few times by pilots.

More interestingly, I have never seen them described as crew rest seats. I have seen flight attendants take all kinds of goofy measures to keep people from sitting in them, particularly on full flights. But they will never mention that the reason people can't sit in them is because they are designated for pilot rest. You would think they could avoid a lot of consternation if they would just make a public announcement about why 21 D, E and F must remain empty.