The last tine my middle seat was filled, a flight attendant offered it to a passenger who was moved out of a regular middle seat to accomodate a family who wanted to sit together. You can imagine the eye-rolling my wife and i did. But, really, we had no "right" to that empty middle seat. We just lost the game. As a Diamond, I could have taken an up-front seat and left my wife to fend for herself in the exit row. I lost that bet that time, but I win it most times.