When I got bumped last year, I asked if it could be rebooked to get miles, and it was done without question (AA). I also asked when a flight was cancelled, and it was done. I think they see it as some of the compensation for inconveniencing you. Unfortunately no extra miles for our recent operational upgrade, but 9000 miles for delaying us over 2 hours in the plane. Not complaining.
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DtG