I think with capacity discipline, UA should offer for FREE the ability to do same day standby for ANYONE on any ticket. I think it would actually save them some money when irrops do occur. If the later flight gets cancelled when you could have put them on an earlier flight or if you can sell the seats on a flight later why not? Why let a plane leave empty? It really seems dumb to me and I think airlines in general are being too shortsighted.
Back before I got status, I wanted to switch to an earlier flight and they wanted to charge me $75. I missed my later flight due to cancellation and got a full refund. UA was out $500 when they could have moved me into an empty seat that left in which they got $0 for.