Most of the FF VI cast though does have a relationship to the Empire.

Terra - Imperial Slave Doll/magic experiments.
Celes - Imperial General turned traiter
Edgar and Sabin - Their Kingdom is in a misleading treaty with the Empire, and they are consipiring with Rebel forces to oust the empires control.

Cyan - His entire kingdom, including wife and kids, were poisened and killed at the hands of Kefka during the war between the two countries.

Strago and Relm - Their introductary arc is there connection to the plot.

Locke - Member of the rebels, as well as the Imperials marching through his home caused his girlfriend to go all coma like.

Gau, Umaro, Mog, and Gogo are the only ones without any real connection to the empire of the cast.

So yeah, the majority of the FF VI cast is pretty invested in the fight against the empire.

Also, in terms of mechanics, FF VI is pretty solid. Not amazing, but solid. On paper, a game like FF X may have the best of the main series battle systems, but it's execution makes the game so laughably easy that there isn't really a need to make use of any of what it has to offer, so I can't bring myself to really enjoy it all that much, so I can't really rate it higher then FF VI in terms of gameplay.

There are the optional boss's, but those are such a small fragment of the game it hardly makes up for the rest of the game. Even VI, being an easy game as it is (Not so much when I first played it, mind you, I was still learning the RPG ropes around this time. ), takes a bit more thought to progress then X did. It may not have had the quality design of X's system, but the execution was much better handled.

Finally, this is a thread were we have basically started comparing one game to another. So it's perfectly valid to point out that all the games we are comparing have the same problem, as bringing up that problem in one specific game does nothing to advance the argument, as it does nothing to advance one side forward.

P.s. FF X and VIII are really the only games in the series I can think of that do a bad job at keeping all of the characters directly connected to the issues surrounding the main plot (And to a smaller extent V, and I and III didn't really have characters, at least not until III DS.)