I recall reading that passengers were entitled to cash instead of a voucher if requested.
Didn't they actually start that after the United thing zahc mentioned? Seems to ring a bell.
When I traveled a lot for work, I would often get in the volunteer queue for getting bumped, though I was rarely chosen. Depending on what they offered. My minimum was a voucher for CONUS travel. I had a girlfriend in DC for a few years, so it was worthwhile for visits outside of the few times a year I was flying to DC anyway for work. I one time got lucky enough for an "anywhere in the US including AK and HI" voucher.
There were times they couldn't have got me to do a volunteer bump for anything, usually after some exhausting TDY work thing where all I wanted to do was get home and hit the rack for a full day. Otherwise I was always open to it.