Why "chance to avoid interruption from stuns while casting" is not very useful.

It's because it's "chance to avoid interruption from stuns while casting" so if you are already in a stunlock and want to lightning warp out of it then tough luck. Alt-f4 for you. If it were "chance of stuns to not prevent casting" or something then that would be more helpful.

If that is already the case then maybe the stuns are queuing when there are too many to animate due to duration.

Well that is whay you have things like Cast on stun, Cwdt and flask whit stun recovery and you know a loot of things that are situational.

What you proposing have no sence, casting is a time duration/animation event, STUN is INTERUPTION whit a time duration/animation.

Interuption is a time event whit set duration, you cant just bash your butons and hope your % RNG to ignore it and ther is noting to say it wont hapen agen before you finish casting. That ther is a Dance Gamge on the duble whit no point.

The thing you loking for is a stun recovery. Stun recovery shorten the animation time of a stun, if duration is to short stun animation wont hapen so you wont get stun.
As thing are now you be just fine whit some stun recovery preferable and a loot of cast speed combined whit "chance to avoid interruption from stuns while casting".

Most dengerous situations are when you are stun when you are in a casting animation.
Let say you have low cast speed and stand on chill graund, you get stun in mid of cast and you will not be able to move until that animation end, if you are just stun when runing you be free after a stun end but if you wher castin you need to finish that animation to be able to move.

I don't think you have face-tanked mobs while trying to kill them asap (like puncture mobs).

Definitely useful, to the point I pick it up when I'm getting stunned a lot. Especially noticeable, when fighting tentacle miscreations (and fast firing mobs). If you don't burn through them quickly, the continuous stunlock can easily kill you, instead of just tanking a few hits and destroying them in return if you successfully roll to avoid interruption.

Yes it realy help, also many people may not know that Energy shield grant 50% chance to avoid stun.
If you have even 1 ES in moment you take a hit you be granted same bonus.
So if you have 10 ES and let say a Jewel whit 2 ES on hit you can basicly keep this bonus constantly.