The magical accuracy for Mortal Ray has been set low.

On top of the effect of Mortal Ray, the fact that you can defeat enemies while they are asleep with it and also considering that blue mages have a heavy arsenal of sleep methods when compared to other jobs, we wanted this spell to be used while being conscious about the magical accuracy when solo or in parties.
I agree the accuracy should be relatively low, but if you set it too low, it won't ever be worth using. Remeber 1000 needles?