Me and Zyphi started a 2 player multiplayer game (although we haven't progress it that much yet). I have my own single player game that I play mainly.
I think the game (although massive) has good replayability. You can just make different decisions to point where its a completely different game. Also you can progress a multiplayer game without the other people - you just take control of all the characters and the others can drop in when they want.
No real advantages of multiplayer - just cool to experience it with someone else, giggle at the decisions and failures etc! We did it on honor mode - wish i think is the same as tactician but with a single save point?