In my very first post, I stated multiple times that YES, YOU PAY 5 STACKS FOR IT. Did I ever deny that? NO. I said in explicit terms that was the cost. You pay 5 stacks for what amounts to one of the best defensive CDs in the game.
Every good tank views their tank stance as an additional defensive CD. Unfortunately, for PLD and DRK, that defensive CD comes at a very high cost so it's their last option. As Layla Bell and Xeno put it, using Grit is like using an IB that deals no damage, doesn't heal you, and then gimps your damage. WARs have the benefit that their equivalent does not have so extreme a cost because of Unchained. You speak of Rampart and Shadowskin as if those two CDs aren't also 2 of the best defensive CDs in the game. Or, that Defiance + Unchained isn't basically the same.
Thrill of Battle boosts your max HP by 20% and heals you for that amount.
Convalescence increases the healing you receive by 20%.
Defiance boosts your max hp by 25% and the healing you receive by 20%. Gaining access to Equilibrium's heal is much better than ToB's healing component because you can control when you get that burst of HP back rather than have it front loaded.
So, come again about how it isn't their combined effect or better in practice.
But really, if you want to look at theory, then let's look at theory --
Do you know how ShO and Defiance compare?
25% max HP increase boosts your eHP the same exact amount that 20% damage reduction does. Do the math.
What is not equivalent is the value of healing. That is why Defiance has the healing boost that it does. All in all, the gap in healing is in the single digits. So, the effective survivability is basically a wash -- a point that people have always said about Defiance vs. ShO. It really doesn't matter that Defiance doesn't heal you unless you are in a situation where you need the max eHP immediately (in which case you can just Equilibrium).
So you pay what amounts to ~370 potency worth of DPS over the duration of Unchained for a defensive CD on par with Rampart and Shadowskin with slight side-perks like a boosted party shield with Adlo + deploy, significantly better enmity generation, and flexibility with self-healing. Also, what people don't often mention is how the massive ballooning of tank HP vs. the lesser scaling of damage has influenced this balance. When you have 34k HP + Equilibrium, you can live a long time with only supplementary healing from HoTs and fairies. That's a big boost from 27k HP + no Equilibrium. Your healers can top you off and then go to DPS town with little worry.