The big deal is that for one thing he may not know it's from that bin door, and passengers seeing it will definitively not know this (unless you saw the incident as you did). Basically loose parts, however small, may indicate that something is not right with the aircraft, and that is always a concern for a pilot. Secondly it is a sight you don't want to leave for the passengers. Doesn't really inspire confidence in the airline now does it?
