If it is a meal flight and they fill it up,but its only catered at 75%,it opens up a can of worms.The FA may not know who gets to eat,the upgrade may honestly not remind the FA when food choices are being offered(I've seen this) and a paying FC pax may get gyped and their will be a new thread about FA/GA incompetence.If its done on non-food segments,the policy will be thrown in the FA/GA's face on the food segments with comments like "They did it on my last flight,why cant they do it on this flight?"

I think CO is correct in not filling up FC just for the sake of filling it up.