A few years ago, my wife and I were flying to SC form Taiwan via San Francisco and Atlanta. We arrived in San Francisco 5 hours before the flight. Checked in downstairs and got our boarding passes. When it came time to board, they said our tickets were invalid. They checked on it and said because of weather, they rerouted us through JFK in NY instead of Atlanta and that flight left 20 minutes ago. We were sitting at the gate the whole time. The gate for the JFK flight was right across from our gate. Not once did they make an announcement at that gate for “passengers so and so to check-in, this is the last call”. We were able to squeeze onto the original flight. They put us in the seats up front with no leg room. When we boarded, there was no room in the overheads for my carry-on. I ended up putting in on the floor in front of me and spent the whole flight with my knees almost up to my chin. Meanwhile, I look across the aisle and the bulkhead seats - with plenty of leg room - are occupied by employees flying back to Atlanta. So much for the paying customer.
It is not just employees who get choice seats, it's their family members and friends.
You read about the guy who tried to open the door to the plane going to China? He was a non-rev passenger who was upgraded to first class because his mother was a flight attendant.
If you are on stand-by or are hoping to get upgraded you have little chance if a relative or friend of the gate attendant is there. Yes, there are rules but they are not followed.