In certain cases, volunteering to be the bumpee (in the U.S.) can yield enough vouchers for many free flights.

Back to the rare event case: I used to take bumps all the time on ORD-MCI flights on the days of the Big Eight tournament. Sometimes they'd need 10 or 15 volunteers to get off a 727 and they'd pay $750 per person. Usually they'd confirm everybody on the next flight, which would also be oversold (but obviously not "to the max"). Sometimes that flight would go over too, yielding another $500 or so. Meanwhile, I'd sit at ORD and watch hoop for an afternoon.

I have taken many hundreds of flights and never seen IDB related to oversell. They always get people off of the plane. The worst possible case - a packed 727, ORD-MCI on the Wednesday night before Thanksgiving - I saw it get to $1000 plus hotel plus dinner plus first flight out on Thanksgiving morning. People were in their seats and nobody was moving as the number went from $400 to $600 to $750. Finally, they got enough people out at a thousand a head and closed the door. I don't know how high they would have gone before IDB - a riot probably would have ensued.

Also keep in mind that VDB is "funny money" (vouchers, which we all love because we are frequent-flier geeks) whereas I think IDB probably involves a stack of cold hard Franklins.