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.
I have heard a rule of thumb is to have a coach at least 400 rating higher than the student. This suggests that at a difference of 400 the higher rated player will have a much deeper insight into her opponents game than the lower rated player has in his own game. That doesn't mean upsets aren't possible, the 2014 Olympiad commentator on Chess.com today said he dropped below 2500 due to losing to a 1940 in his most recent game.
Off the top of my head I believe 700-750 rating points is the point where ratings don't change. So, at about a 700 point difference the superior player should win every game. Granted, anything could happen in an actual game.
A beginner has about a 1 in 40 chance of playing a gm move by accident (given 40 legal moves in a position). Its not inconceivable that they could play several really strong moves in a row and end up in a very good position. At the same time, a very good player could be distracted or overplay the position and play some weak moves. Anything's possible.