Personally, I think Lore makes a game great. That said, a game with good lore does not automatically make it a good game. Conversely, a game without good lore can still be a good game.

In the context of MMO's, I find FFXIV's lore to be thoroughly underwhelming and uninteresting. Despite that, it's my most played MMO.

Conversely, I really loved the lore of Wildstar, World of Warcraft and Guild Wars 2, but due to other reasons (mostly gameplay related), I don't play any of the 3 actively.

In terms of adding content, it depends on the depth of the content in terms of how much lore will matter. As the previous poster said, Yokai event is obviously not lore-friendly, but it also wasn't terribly important to the core of the game. Jobs, on the other hand, are. I think it's important that the devs stick to the lore for core things, otherwise it ruins the credibility of both the content and the lore.