Airlines always overbook on the assumption that a certain fraction of folks with reservations won't show up. They have excellent historical data in this regard, but - unless the reason was something like a delayed inbound flight on the same airline - they don't know why, and they don't care. All you're doing by not showing up is giving them another data point for the future. That person who desperately wants to get on the flight already has a ticket, will be at the airport, and will get on the plane if you don't. Please don't worry about a poor soul being stranded.