Why not surprise and delight your best coach passengers with something that costs little and leads some to chase status, spend more, etc.? I think where it becomes a problem is when people expect or feel they are owed something they didn't buy or where it creates scarcity for customers who did in fact buy it. In my view if a paying J customer has IRROPs and needs a seat on a recovery flight they should oversell and bump someone who didn't buy it too, but I suspect that wouldn't be universally popular.