To be fair some of the grumpy pax are connecting from a sleepless overnight intercontinental flight and are just tired. It's hard to join in the small talk at 6am when you havn't slept for 24 hours.
But the check is necessary. I was once on a plane where four people were fighting over the same two seats. It turned out that two were on the wrong plane.
And it *is* necessary for some people to be told which number their seat is. Some are just stupid but many more can't read the number without their glasses on. Everyone should follow the example of Dnata at DBX who print the seat number VERY BIG. Airplane makers could also speed boarding by making the seat numbers more readable. On a Fokker F70, for example, short people can't see the numbers unless they stand on the seat.