Of course you guys want everyone to 'play to win'. If everyone else is 'playing to win', it gives some measure of predictability to how others are going to act: always in their own best interest. Therefore, it gives you some measure of comfort in how YOU can go about winning.
But, if you're only requirement is that it tries to win, you might see the Mercs again. I'm dubious as to how it could work in Civ V at all till I see the game, however.