According to who? The GCBTW? Sure the devs put a great amount of importance to the story but it's one of the many, many things the game has to offer. MMOs are inherently not single player, you CANNOT play XIV's story as a single player game simply because you need to use DF.
Do you know what an MMO is? A game isn't either an MMO first or MMO second, it's either an MMO or it's not.