There's different levels of stories - I like little details, little comments about people and places as you play which give a feeling of a world with depth. The over-arching story is not so important for me.
I kind of put those in back story category. That kind of deals with the momentum of a game. If you do those things without a main story, then there is some other goal required. It might be missions, or simply leveling up, but you have to do something to push the character forward rather than just having them interact endlessly. The game is definitely more simulation at that point. I played a space trading game like that without ever engaging in the main story. I think I played it about the same length of time if I would have engaged in the story. The only trouble was I just quit because of boredom. I had leveled up to maximum and there really weren't many challenges left. It didn't have that satisfying conclusion. Most of the time, with role playing games, I never make it that far, anyway. The game play becomes too repetitive for me to bother finding out what happens with the story.
I think a main story is a separator for games. They are going to all start looking very much alike without a main story. There's only so much you can do on the game play side of things. You can trade, fight, or solve puzzles basically. Puzzles can only be solved once, so they are in a slightly different category.