I recall back in the late 90's, the company I worked with flew a lot of people BF from EWR to MAN in the UK. On summer flights in particular, CO was always upgrading people in coach to BF. It caused resentment among people in the company: they didn't talk about BF and the service, they griped about tourists on $500 tickets getting into BF. Surprisingly, even people flying internationally for the company for the first time whined about it. I am sure that they exaggerated, but I remember one guy claiming that CO had given upgrades to 14 or 15 people. I can attest that, among some, the free upgrades creates resentment.

I did sit beside a lady once who had been upgraded. She was flying on a heavily discounted ticket. She used to be a flight attendant in the UK, but CO didn't know that. We got chatting. She told me that she was called over and asked not to tell anyone that she was a coach customer getting a free upgrade--that part of the plan didn't work. I frankly was happy for her good luck.