So, according to the JP Community rep Oriole (click for link to post) which was translated by Slycer over at BG this is still a crappy SP, but they apparently have a master plan behind it:

Regarding the New RDM SP Ability

Thanks for your feedback. I'm going to respond to some of your opinions about the new RDM SP ability.

Leviathan.Niuuu (JP Player, pix) said: »
My only thoughts are on the RDM SP ability. Seriously, just one spell? It's like a crappy version of Elemental Seal. Enfeebling magic is weak as it is...maybe either allow for more casts in a shorter duration, or potentially to allow the spell to ignore monster immunity.
Elemental Seal significantly increases magic hit rate, so it can still potentially be impacted by a very highly resistant monster. In the majority of situations where it is difficult to stick an enfeeble, it is because the monster is very highly resistant. In order to be able to stick the spell and ignore the resistance (bypassing Immunobreak), the new RDM SP ability can be used to give the debuff reliably.

As far as immunity goes, we are planning on tuning each monster individually and getting rid of immunity as much as possible. In Voidwatch, Legion, and the Limbus/Einherjar high level expansions, immunities have already been reduced to almost none. We will continue to perform these adjustments to other monsters as well as in all new content.

Regarding the suggestion to allow for spells over a duration instead of just one case, although we have not completely disregarded this option, we want to adjust very carefully as a monster could be quickly debuffed completely and then Stunned continuously for the remaining duration of the ability. We'd be grateful if you'd test out this ability first and then let us know how you feel about it. {{This just in: Stun has a recast timer unless you use Chainspell}}

Also, about your opinion that the effects of enfeebling magic are too weak in the first place, we also want to give enfeebling magic a more noticeable effect and we would like to tackle this in the future as a separate issue.

Phoenix.Ploplo (JP Player) said: »
About the RDM SP ability - will it wear off if the target has total immunity to a spell you cast? That would be kinda dumb, although I guess it can be used to check if a target is immune first.
When an enfeebling spell is resisted completely, the effect will not be lost and will be carried over to the next enfeebling spell. By the way, if a monster is fully resistant to something, the log will have a message that says the target "completely resists" the effect, so you can check this first.