Overbooking still makes little sense for jetBlue given its current size. Maybe it could work in the NYC-FLL market when there are multiple flights but in many markets there are only a few flights a day at best with no other airline alternatives to rebook people on-without B6 forking out cash to pay for a ticket on another airline. This combined with the fact that the regular $150 cancellation fee is equal to their average fare means that they are getting the revenue even if the person does not travel. Yes there are a very small percentage of people that may be travelling with refundable fares or are mosaic customers who cancel at the last minute--but this is not going to have any significant impact on revenue.
If B6 oversells a flight from SFO to AUS, the pax has to wait a full 24 hours to get the next non-stop plus B6 would potentially need to pay for a hotel, etc. It is very rare that flights are completely sold out and when they are, it is often due to weather or other circumstances which means that people from cancelled flights are filling any available B6 seats.
I just don't see how overbooking could benefit B6 in a significant way. Having worked at a legacy carrier which overbooked flights to the Caribbean by several hundred per flight during the holidays--leaving half the people stranded, I can tell you this does not endear the airline to the passengers that were intentionally jerked around.
There is also no proof whatsoever that airlines offer lower fares because they overbook. Legacy airlines drove themselves into bankruptcy with their high costs and crazy fare rules. They charged a fortune for many last minute tickets in markets with no competition. B6 offered one ways requiring no Saturday night and many markets have seen significant changes in pricing over the last 20 years. This has nothing to do with overselling making it cheaper to fly. The airlines are trying to make more money--oftentimes at the expense of their customers.
I really doubt the execs at UA/DL/AA are sitting around thinking that we can give everybody a rebate because our of massive overbookings over the past year. They are hopefully figuring out a system whereby the overbooking is not costing them more than they are gaining. In B6's case, they would be more likely to lose more than they would gain.