The only advantage for signing in I can think of is that two players are able to equip their own skills (only true till you complete one campaign thus having 8 skill sets) and spend their own points on the skills which they prefer first.
Disadvantages: - Player 2 starts with 0 weapons, items and SPs if not already played before. (especially the SP part is important to me as Ive already played mercenaries a lot and thus have many points) - Buying Skills with 2nd Player is kind of very tedious (unless Im stupid and didnt find a more simple way?): you have sign out with player 1. Press Start with Player 2, buy and equip your skills. Sign out with player 2. Sign in with player 1 and press start. Sign in with player 2 again. I couldnt find any other way to buy skills with Player 2... have I missed something really obvious or is this really the only way?
So advantages for NOT signing in: - Skill Points wise, 2nd player is always on the same level as the first player - Save data for both characters gets saved on the same account, thus if you want to play with the secondary character in single player you have the items and so on acquired in Split Co-op
disadvantages being: - both players have to equip the same skills until 1 campaign is finished - Same SP stash
So Im sticking with not signing in. But I wanted to know if I missed any advantages for Signing in?
And also please do not that while the game does say that by not signing in with 2nd player your progress will not be saved, this is not true. It is all saved on the first player´s account.