Citadel Buster is not the best example, as that particular ability was NOT random. It was on a strict timer, used regularly starting once the boss reached 20% health. (Also, it was actually Proto-Ultima that used it.) Armor Buster is a better example - while not as lethal as Citadel Buster, it was still pretty bad, and Ultima randomly chose between it and several other TP abilities. While the odds were against it, it COULD use Armor Buster several times in quick succession, and doing so would generally leave an alliance in tatters.
This was pretty common in FFXI; bosses would typically have at least one really nasty trick, and it would be randomly shuffled in with a bunch of less nasty moves. How often you got the bad one was pure chance; you might go the whole fight and never see it, or the boss might use it back to back.
The thing is, though, bosses in FFXI were reactive, not proactive. They weren't a matter of memorizing patterns, and preparing yourself for attacks that you know are coming, so much as learning what to do recover after the boss took certain actions. It's a different sort of playstyle than we're used to in FFXIV, but I don't think it's a bad one. Weathering unfortunate boss RNG of that sort was among the greatest challenge that the game had to offer.
The thing is, sync adds an element of challenge to these fights, without making them nigh-impossible. The fact that the option exists to crank up the difficulty to max doesn't invalidate the satisfaction you get from cranking up the difficulty merely to the point where people have to actually pay attention to the fight.
You'd be hard-pressed to find someone who will claim that they gained any sort of satisfaction out of completing a fight unsynced, while plenty of folks will get a rush from doing the same content synced, but not minimum ilvl. "Why bother to make it hard for yourself, when you can make it SUPER hard for yourself?" is a very thin argument.
I understand and respect folks who want to get a taste of what the fights were like back when they were still relevant.