The thing here is... joining an FC doesn't take you out of the community at large. It's tantamount to saying, "I joined <some organization> and have completely stopped talking to my friends and neighbors who aren't part of it." FCs and Guilds exist for 2 reasons... first it's an easy way of grouping up with your friends and having a consistent set of people you know will be there to help you if you need it. Second, a majority of people like feeling as-if they belong to something bigger than themselves and the act of contributing to those things makes them feel good.
Now the perks you get for being part of the guild are great (FC housing, the exp bonuses and the like), they were added to further incentivize joining a group, but the over-all mindset at SE has been that content in an MMO is meant to be played together with people you know. This is the core of gameplay for them in their MMOs and you could see this especially in XI.
What I don't agree with for what SE has done with it is how they've gated off some content behind having a large, thriving FC as the only way to access it... Personal Housing, Gardening, Chocobo 'Raising' (which is completely not the system I was hoping for, by the by). Anything where you need one of those inordinately expensive FC houses to do, which if you have a very small FC become mountainous underdertakings to achieve.