Getting bumped is usually a bonus - I've had it happen to me a few times. I volunteered for it once - plane out of Miami was chock-full of about 200 out-of-control, hyperactive, rowdy and under-supervised young / teenage Latin-American kids going to some soccer tournament (think behaviour at a typical Brazilian soccer game x10, in a confined space...). The gate agent was sympathetic when I volunteered to be bumped off that flight - she knew what I was really asking ("please don't stick me on this plane ride from hell for the next three hours") so made it happen for me with a wink and a nod. Made a few extra bucks too as I recall.
Definitely can be a good thing sometimes!