This goes back to what I was talking about with allocating resources. Unless you're a super-hardcore power leveler who will spend hours of time on optional grinding, you're going to have a limited amount of resources to attribute; for example, you can't give all of your characters in FFVII KotR and Bahamut Zero on an average playthrough without power-grinding, nor can you give all of your characters in FFVIII the same GF abilities and same magic junction allocation. Usually, each member in your current on-field party will be functionally different from the other two.

Now, it is true that the off-field party members will act as carbon copies once you transfer all of the statistics from your on-field members to them. However, it is only the characters who are currently on-field that matter, and the characters who normally remain off-field will be the characters you don't like.

So, even though the currently off-field members will function as carbon copies of your on-field members when (And if) you decide to bring them on-field, the characters on-field will still function differently from eachother. To strengthen the "on-field characters matters only" argument, in the more class-based FF titles that had large parties extending your on-field supply, such as FFVI or FFIX, you could still get away with using the same on-field characters throughout the game without switching out for the off-field ones.

That way, there's variety among your on-field group, and you can get away with using only the characters you like.

Sometimes it's bad enough having to see certain characters (Again, Cooke and Mack; don't know if you played Lost Odyssey or not).

True, it isn't unreasonable to use a character you hate if they're good in battle, but I believe it would make it better to not be required to use that character at all.