How large can a rating difference become before either player realizes it's not worth engaging in a game? I think the relationship changes as rankings increase. For example, players with ratings lower than 1000 might be able to play all other players in that range and still enjoy themselves, but if a sub-1000 player matched up with a 1400+ player, the two might not enjoy themselves in the same way. At which levels do players begin to learn strategy and stop making mistakes?
One thing is, I think there may be real surprises even if the rating range is quite vast. You'll never know the real strength of an opponent most of the times (because of lack of rated games etc.,) On another perspective, lets say one of the player is 1400 Elo and the opponent is 2000 Elo, it will still result in a good "feeling" for both the players if they take it sportively. The 1400 Elo player will clearly appreciate the consistency in the 2000 Elo player's moves and will really learn a LOT. The other way round, it is a good opportunity for the 2000 Elo player to test out his skill or experimenting things which he would not dare against a player of his own caliber (Like for example sacrificing the Queen for a Knight and Bishop and trying to win in the given situation).

So I feel there is no rating difference that can make the chess game not worth engaging, if you look at the bright sides.