If your team losses you get an iron medallion. You can have up to five iron medallions on one character. The next game you win, the game automatically redeems the medallions for (if you've don't the NF) 44 reputation each. For the win you get 55 (with NF) so leaving actually costs you more reputation than if you had stayed and lost. Also losing after you have 5 medallions will wind up costing you rep because you can't have more than 5 of them at one time.