Ultimately, the only thing that is important to a game is the experience that the game is designed to create. If the game doesn't need an elaborate story to bring its experience to life, then that's entirely okay. Tetris doesn't really have a story. Most Mario games don't need much else than a brief "Oh no the Princess got kidnapped again!" premise to get it's ball rolling (the Paper Mario series being a strong exception to this).
But if your game is trying to have an interesting story, then yes, Storyline absolutely matters. Not every gamer places value on it, which is entirely fine, but the way I figure it, if you're going to try to have a story, why the hell would you screw it up?
Your friends are probably uncultured brutes who haven't experienced a game with an engaging story, so really you can't blame them.
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This is pretty much what I think. And yes, they do seem to lack knowledge in these things.