In order for an MMO to be successful all types of content is a must. Because this is how you attract and keep an audience.
Nobody (or close to nobody) just raids. The player might do beast tribe quests on their free time, or chase ponies. Nobody just crafts. They might be interested in gathering, the gold saucer, or PvP. And that is what keeps the game good. Because when you're not doing your primary focus, you still have something to do.

That being said, the previous guy mentioned Diadem. Here's me in Diadem yesterday.




Before diadem came out, I didn't do fishing that much. Fishing on a timer with a possibility of a chest spawning and not bad profits though really intrigued me, so it's my main source of income right now. My free company mates actually go and kill in diadem. Now that there's no huge crowds they find it exciting. They like to challenge themselves and see how many people they require to kill specific ranks. If diadem never existed there would be one less thing for them to be excited about.

And that applies to all content. If we got a very limited range of content that "one less thing" would add up. Until that MMO next door would have one more thing. And would end up being so much more appealing.