It's written into the system for me:

Airlines overbook flights. They do this for ONE reason, they profit from it.

I'm not going to waste time and money on informing an airline that they are free to sell my seat again with no benefit to me on the off chance that:

a) the flight is oversold to capacity; and
b) someone may subsequently need my seat that mysteriously is unavailable on any other carrier/route.

If it could be done via MMB or there was an incentive (partial refund if seat resold) then I'd be inclined to do it. Another option is some miles as an incentive, you have paid for the flight.