If you arent training you bench players, earning money, or getting "something" in return besides the possible injury - why join a private league?
which possible injury, every injury is healed at the end of the game, because it is an arcade game.
And it is fur fun, to play against opponent you like to play, but normally aren't in your league for example friends who playing buzzerbeater also - such wins could be more improtant then one in a league :)