It is cause to dismiss it when there are alternatives to it or if the mechanic isn’t necessary. Hitpoints might be less dynamic than fatigue, as your chance of getting stunned would remain stable for most of the fight, but would make more sense thematically and would give a boost to the stat and Colossus perk. It would also remove the problem that undead pose for the fatigue approach, since they never get tired and thus would always get a boost regardless of how much they fight. In contrast, locked spread or not, making it fatigue based would only further cement the superiority of high fatigue characters over low fatigue ones, as for its effect to be dynamic the variation between stun chances at the different stages would have to make a noticeable difference, otherwise why bother, which would result in large differences between a high and low fatigue character. Fatigue simply does not need a buff like that, therefore I’d rather see the chance of being stunned depend on HP, if the decision is made to tie it to a stat.
That arbitrariness is just the way the system works, similar to how none of the stats increase the amount of damage you do with weapons. That’s arbitrary, as you’d expect individual strength and skill to be a huge factor, but it’s done that way so as to shift your attention towards the gear, rather than the stats. Same goes for stunning, which is part of why I’m against having stats influence the chance to stun – that should remain the territory of weapons. I’m inclined to think the same of the chance of getting stunned, since it’s not all that common and only really a factor when fighting Orcs, which is fine and something that makes them different from other factions, while still being manageable with the right preparations.