I thought DL was doing the 'auction' system - where you can tell them how much you want for a DB at checkin, and then they will take the lowest offers. Obviously, if there isnt enough demand based on that, they'd have to go the traditional offer route. Or did they abandon that approach?
UA does offer the overbooking notice at check in, but that just puts you on the list of interested folks - you don't get to suggest an amount, and how the GAs even use that is kind of uncertain.
Coming from a non-hub, so nearly always needing to connect, I'd like to see UA specify which of my segments is overbooked. It makes a difference. If I'm doing SFO-ORD-SFO, it makes a difference. For example, if its the first segment, I'd like to know, especially if its the last flight I can take that day, because if it's the last segment, I'd like to know I'm closer, with just the last hour left to go. An overnight delay out of SFO means I can't get back until mid-afternoon the next day, where if I can get to ORD first, I may be able to DB off the last flight, but still arrive faily early in the morning.
As to proactive DB comp, I don't see that happening except in rare circumstances where they know they overbooked (or in case of capacity downgrade last minute - i.e. 320 goes to 319). There is a difference between a flight being overbooked vs. oversold. The former means more seats are booked than are on the plane, the latter means that more people are checked in then there are seats for. The reason to overbook is because they want to sell tickets to make sure the plane is full, vs. having X% of tickets booked and don't show up, leaving those seats empty. They don't know how many people aren't going to make the flight until at least the checkin deadline, and maybe even later because of potential misconnects. There is a balance, they don't want to pay DB comp. to folks if there are seats left for all pax that show up.