Advantages:
-This allows you to play with a friend, and you can still play on it singularly on the same file.
-Theoretically, Your weapons ought to reach lvl 10 quicker ^^
-you can tell eachother exactly what you want and tactics.
-Finally, Its fun!

Disadvantages:
-When you have to have a player do a bot job, and they-re rendered defenceless, and this can be irritating if it means a player has to go back to protect the other.
-2 players instead of 3.

It is slightly won over for co-op, but they are useful for different situations, and take advantage of the great amount of the andvantages ^^ =) ^^