My friend had to clue me in on this, as I had never used the Grimoires on Zeromus EG before. Using them will result in a strange bug (for some of them) that just performs a seemingly random spell animation (that is not the animation for the Grimoire), and may turn ZEG into a toad, then have him take 9,999 damage after awhile and turn back to normal, or just have no effect. It's too bizarre to have been intentional.
Thankfully (and I hope this is true), most of the bugs have been fixed for the European versions.