FFXI has a theme, it's deception.
Buyers are deceived into thinking they're getting the new high-quality FF game, and find out it's online!


Also, to continue on feioncastor's point:

FF8 - Relationships
FF9 - Reminiscence
FF10 - Religion
FF12 - Revolution (as far as I know)
FF13 - 2B Released soon on PS3!