In most Role Playing Games, a player starts out with a certain party and ends up gaining new allies as they progress through the game. Are you the type to keep the old or switch them out for new allies?
I know for me, I usually keep the old. It's just how I am. My ending Final Fantasy VII party was Cloud, Tifa, and Barret and as I'm currently playing Star Ocean: The Last Hope, my party remains Edge, Reimi, Lymle and Arumat (only because the green-haired guy left).
I don't think achievements or trophies should be created that require you to play online with somebody. One day somebody's going to get a game three years or so after it releases and is going to expect to get all the achievements, but little do they know, the game's achievements are based on the game's online mode, leaving them suffering because there are 0 gamers playing online.