I use my pen on flights (in plain sight), for written work in general.
On one flight, after seeing that I used it to complete customs forms, my seatmate asked to borrow it to do the same. I said 'yes', as I enjoy helping others. After my seatmate finished, everyone else on my whole row asked to borrow it too. I still said 'yes', to not appear selfish. Then one borrower sneezed mucus on my pen that she wiped with her shirt, but she apologized.
On another flight, a borrower unscrewed my multicolor pen "out of curiosity", and broke it.