This practice creates so many shitty situations.
Last year I arrived for my flight to discover my aisle seat with extra leg room had been given away and that I had been moved to a window seat near the back of the plane. I explained that I purchased the extra leg room because I have serious back problems and need to stretch out, and that at the window seat, in pain, I might experience a claustrophobic panic attack. They told me in no uncertain terms that I could take the new seat or get bumped to another flight. I needed to get home so I took the seat.
I was told my seat had been changed due to a "mechanical issue." Boarding the plane, I noticed my seat was occupied by a United employee.
As the plane began to taxi, sure enough. Had a total panic attack. Just flipped out. They had to stop the plane. Everyone hates me, makes it worse. They asked the guy they gave my seat to to switch with me so they could take off. They ended up going up and down the aisle just begging everyone to switch with me so I could at least sit on the aisle, asked that guy multiple times and he refused. Eventually found someone.
They never even refunded me for the leg room charge....said they would but never did despite multiple inquiries.