Hi, it is absolutely true that not all airlines do check passes at the door.
In fact in Latin America none of the ones I took did, even on intercontinental flights. Same in Europe. So it's quite possible that many pax don't know that they will be asked. I try to remember which ones do ask and check the preceeding people to see if I need to show it.
I think airlines that wants no delay in checking passes at the door should have the gate agent remind pax to keep it handy.