If you're lucky though, agents have the discretion to assign a confirmed seat for free instead of a standby on a later flight when there's a lot of space available.

I missed a flight from BTV and was panicking when I got to the B6 desk, and the agent said, "Oh no! I'm so sorry, but I have some bad news..." [Oh no. This is going to cost me $$$.] "... it's five hours until the next flight. Here's your new boarding pass."

That flight was very empty though. It was an A320 and I was in row 6. They let me move up to the EML seats in row 5, where I had all three seats to myself. That was nice. With 38" of legroom, being able to put my drink on the tray next to me instead of in front, being able to see the map and watch TV at the same time, and with no one immediately in front, next to or behind me, I couldn't help but think that it beat a lot of airlines' domestic first class. Too bad I was only on the plane for 90 minutes!