Ok, stop. This isn't up for debate. The dev diary tells you straight up that "your chance to fully avoid the attack is equal to your Block chance plus Parry chance plus Evade chance". That's how it works, it adds them all together and rolls against that. The same method applies for partials which we were able to prove on that other thread I linked by getting a character to 100% combined partial avoidance chance and observing that said character was never fully hit during a 10m parse of multiple mobs constantly attacking them. If avoidance was three separate rolls against each avoidance individually instead of a combined roll that character would have received quite a lot of full hits. So we do know 100% how BPE works.
BPE is working as intended, it's just getting devalued by a number of factors. The main one here is just how much rating you need to get to reliable BPE rates, this is the ratings curve for full BPE chance:
To cap out full avoidance chance on your character panel you'd need ~630k block rating, which is pretty insane. That's actually not even enough block rating to truly have 13% either as mobs in instances have various amounts of avoidance penetration that effectively reduce how much rating you have (but you don't actually get to see this at all). You're complaining that BPE is "very RNG" but it's designed to be RNG, having say, 20% block chance doesn't mean that each fight you'll block 20% of all incoming attacks, it means that given a large enough sample of attacks you'll eventually average out to see ~20% of them were blocked.
So yeah, of course it isn't worth stacking avoidance on your Captain right now, you'd need to stack a ton of essences and it would only serve to make you take less damage from physical attacks which aren't necessarily all the notable in an encounter. You're trading in a universal defensive stat (morale) for a situational one (avoidance) and you're doing it at a terrible exchange rate.
Now you've suggested several times around these forums that "fixing" BPE would change the meta and bump Guardians up in value compared to Captains but in reality it wouldn't do much. Captains gain about as much from improved BPE as Guardians do (admittedly not quite as much as Wardens do). It's kinda one of those things that affect almost everyone to the same degree, it ain't gonna change the meta; it's just going to maintain it.