My experience, and I fly TWA a lot, is that a bump only happens about 1 in 100 flights. Mostly on busy Mon or Fri or special events or special holidays. I think reason it is much less than before is that 1) they do not overbook as much to keep hassels down and 2) more people standby for earlier flights if they finish their business early. This, of course, opens up their seat for a later flight. The "policy" that one can take an earlier flight w/o any penalty cost (for those with restricted tickets) is a smart move for airlines. It gets someone on an earlier flight IF space is available and, therefore, opens seats on later flights which help for overbookings, cancellations, delays, etc.
I bet in 8 out of 10 cases where they even took volunteers, they did not need them because enought confirmed passengers were no shows.
Some flights (like to Honolulu) used to be overbooked most days - but not sure that is true anymore. And as people don't like to delay vacations, bumps were good deals as one could bargin for a "system wide" voucher.