I'm pretty sure in the original game (or possibly it was just in G&K), a city state would vote for you for the rest of the game in world leader elections if you were their most recent liberator, regardless of whether you were allied with them. All other benefits (luxuries, resources, etc) went to whoever their ally was, but the liberator always got the vote for world leader.
I am not sure how it used to work (or how it currently works) for regular civs...I *think* you were also supposed to get their vote for world leader, but I am less certain, as I never actually experienced that one. I have freed a formerly captured city state in G&K.
I haven't done either in BNW yet, though; curious how it is.
Obviously, at first you get their votes due to Ally status, but I am wondering if you always get their vote still or not?