If you have a game with a great story but gameplay is horrible you'll have a crappy game and won't want to play it since every action you take in the game is bad. A game with great gameplay and no story is still a great game. A game with great gameplay with a crap story usually feels disappointing when you get through those important story points, but can still be enjoyable to do the mechanical actions which comprise the gameplay.
I think the (very critical) point you are missing here is that the discussion is not about games generally, but rather very specifically about role-playing games. The argument you are making may be applicable to some other types of games. But no way does it apply to role-playing games. A role-playing game, by definition, is about role-playing a character within a story. If you don't have a strong story within which to set your character role-playing, you don't have a role-playing game.
