The "Cannot be stunned" -mentality, is it even necessary?

it's not necessary for dual totem CI build

Cast when Stunned, it really does help us CI people. No level barrier like cast when damage taken either. It just means that we have to invest ALOT of links into auto-triggered defense, which leaves barely enough space for 2 main skills and a 3-aura+reduced mana link.

Tanks, who, by definition, are taking paltry percentages of their ehp per hit, naturally experience stun less severly than 'non-tank' characters. If you designed your character around eating damage, and not manually avoiding damage, then naturally you dont mind being stunned and eating damage (except for the leech cases where actively attacking is providing your ability to eat damage as pointed out by scrotie).

Characters with large life pools and large reductions of incoming damage will benefit less from expending points to prevent an already mild mechanic.

Characters relying on misdirection or moving out of damage or can be stunned from almost any damage source, benefit greatly from the ability to avoid stuns, as the stuns are going to be more frequent and of higher duration and more thoroughly negates the planned defenses.

stun continues to be a leading source of desync. Resync when you get stunned is fine if you are heavily desync and then get stunned. It does almost nothing for being pretty well sync'ed up and the sequence of events diverging because the stun occurred. So the resync on stun will help you realize that you were severly out of sync, but does not help you when it was what caused the divergent states, you will need to be stunned again at that point.

I struggled with this exact question for a while since it was important for my build. I tested three configurations extensively:

1) No stun immunity but using Heart of Oak
2) Stun immune via Eye of Chayula
3) Take Unwavering Stance

In the end I decided on 3.

There are several situations which can be very dangerous without stun immunity. While a large percentage of gameplay does not need it, once you get into the 80s and/or you're hardcore, a single death is really painful so you want to cover those situations.

The situations that led me to feel the need for stun immunity are:

1) Any vulnerability map - many hits have stun potential due to decreased stun threshold
2) Charging enemies like rhoas and blackguards have guaranteed stun regardless of damage
3) High damage elemental attacks like Piety's lightning bolts can stun+shock lock you and kill you before you can run away