When you select "Battle" you are saying that you are ready to be a teammate to the other players in the game. While yes, real life comes first and should unexpected company appear then it can certainly pull you away from the game... but please don't think that leaving your teammates is acceptable as a normal practice.
If you make a habit of it, you can be restricted to Co-Op for a set amount of games and unable to join other modes until you've completed them. If it becomes an egregious habit, other tougher sanctions can apply.