Aside from all of the other reasons, GAs moving people around without asking assumes that just because I'm on a single passenger PNR, I'm traveling alone. That might be an OK assumption most of the time, but not 100% of the time. I travel pretty regularly with a companion who is often coming from/going to a different origination/ultimate destination, but we will travel together on the long flights in the middle - obviously on different PNRs in this case.