So... have you used all the classes?

Simple question, have you actually used all the classes before or do you stick to certain roles only?

I have used them all before, yes.
But since I only replay the NES version I'll never use a BLM or THF again.
The other characters I mix and match depending on how I feel. Generally if I'm just going for a playthrough I'll use FI, BB, RM, WHM, but some of my more interesting parties were FI, FI, FI, WHM and Fi, RM, RM, RM.

I have not. I always pick the same 4 :P

I did. I believe for my first playthrough I picked the Warrior, Thief, Black Mage and White. Then for my second playthrough I picked the Warrior, Red Mage, White Mage and the Monk. The latter is my preferred team of choice now whenever I play the game again.

Never really used the poor old Black Belt much. Certainly never to completion. I know he becomes a champ but I just don't have the patience to see it through.

Not even close. Whatever gets me through is good enough for me.

In my last playthrough, I finally used the Red Mage for the first time and thus completed using everyone. Overall, I feel that Thief is probably the only class I will never go back to unless I just want to mix things up. Ninja is not worthi dragging that dead weight around for the early nasty dungeons.

I really prefer the red mage's utility later in the game over the ninja's slightly higher damage. It's cure and cure2 spells combined with the knight's cure spells mean saving the white wizards higher cure spells for chaos instead of using them up on the way. It's more enjoyable then throwing a fight for 7-8 turns so you can heal up with cure staffs and heal helms.

I think I have gotten 1/2 way through the game with a Monk in my party, other than that I have beat the game using all of them.

Of course every class but fighter and red mage has been relegated into the useless bin for me. I can find ways to make them contribute, but with the weakness of magic it doesn't seem to make much sense to use anything but the two.

^
I'd say that's the sad truth of the matter.

I've used all of the classes, but I generally stick with a fighter, black belt, black mage, and white mage. It may not be optimal, but it's the group I like the most. :)