I volunteer almost every time where I have any flexibility, as I do a fair amount of personal travel and love being able to use vouchers. However, I very seldom actually get bumped, as they end up with no-shows, etc.
This week I volunteered for VDB on a flight and they thought they would need me. However, they had 3 no-shows, and I ended up being the last one on the plane. No harm done, but I would have liked getting the voucher, and my travel plans were flexible.
If this system had been in place, I probably would have received a voucher and taken an alternate flight, and been happy with it.