Ran some tests with this and commented in the Red Mage specific forums. I agree that this ability is lackluster, and adds next to nothing to the job.

I can think of a handful of situations where the current incarnation of the ability (where you can land enfeebles on immune mobs) could potentially be useful. However it has been stated that this is a bug and will be corrected.

Simply, I do not think this ability is 1hr material.