In fact it's not that low.
Since it can trigger on all auto attack and physical skill.

The only mistake people do is casting it for a physical tank buster...
When it should be cast long before that and still be up for that big hit.
Because the longer it is up and the more the target is hit the higher the chance is to reduce damage.

It's not an oh shit shield more a preventive spell.

And it should be used as often as possible because it's great ! X3
And also yeah... It's a free skill to add !