Let me rephrase: if a boss has one ability that's considerably better than others, but only uses it very rarely or in a specific cycle, I'm not normally inclined to allow it. The brokenness of the ability shouldn't matter. The same thing would apply for, say, a boss that does .5 PCHP damage very rarely, and .2 PCHP damage much more commonly. Spamming .5 PCHP damage is hardly broken, but it isn't how the boss fight works in game and allowing that wouldn't reflect the true easiness of the fight. On the other hand, some bosses will use their big damage all the time (say, Zeromus, who uses Big Bang pretty frequently), and I have less problem with letting them use it as much as they like in the DL even if that ability is awesome.
I will grant that Jade is an edge case since he theoretically can spam BoltX if he gets lucky (If it's a flat 20% chance as theorized, it will occasionally happen), and can see why others would allow that. Something on a firm cycle like Chaos or False Althena, though... if Chaos could cast Cure4 every other turn he would, and same with FA's big damage every turn. The fact that they don't implies that they can't. (Okay, actually it implies that the game designers are not sadists, but "in character" that's the best reason.)
Heck, I'm more kneejerking Seymour here, but if Seymour doesn't get the drop on FA on turn 3 (not sure which one is faster), then I can certainly see him dying to 2 Rays + something. So it is a close match anyway; BoltX/Ray every other turn is still pretty impressive.