I would have thought upgrades at window would be a little bit more diffcult especially for silver since companies often buy Y & B fares on shuttle fares. I used to get these through mine for the 'flexibility' despite telling my company I could fly free standy anyway, but they never worked it out and paid $500ish for BOS>DCA every flight.

I flew first every trip, but never had to rely on the gold/silver window.

FWIW the donut is horrible and handed to you as you board with your coffe and carry on in hand. First class doesn't get a better donut so don't hold out for that