They may try to minimize overbooking, but stuff happens. We were flying SJC to EUG via LAS today, at LAS they were looking for volunteers due to a previous flight cancellation. We volunteered and in exchange for getting into EUG at 4 PM instead of 11 AM we each received an $800 LUV voucher (that was the offer, and I didn't feel the need to negotiate). So, if you have the flexibility, it can be a nice win-win.