The United seats do earn miles (I thought, I know it does for others), they just don't count towards status.
As I said I don't have a problem with it, but the logistical difficulties are obvious. They can tell you to do it ahead and refund the extra ticket if the plane isn't full, but taking payment once on the plane is going to be difficult.
I'd also be concerned about differences between seats on different planes. Options are provided for point of departure but if changing planes halfway could result in going from a barely fit seat to a not-fit seat then the "just get off" option isn't so acceptable any more.
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