I suppose its effectiveness really depends on how readily one is able to switch between black and white grimoires. I mean, in my experience way-back-when, I was generally relegated to either the main healer or pseudo-nuker in the party, so grimoire switches for such purposes would definitely be limited. However, either way the RDM always ends up trying to land enfeebles. Hopefully the stacking of the JAs will allow the switches to be frequent enough that the effectiveness of certain enfeebles don't take heavy penalties because you can't switch to the appropriate grimoire.