15 skill isnt 5 levels higher. You get more than 3 ninjutsu skills per level up later on. However, as you've stated ninjutsu skill IS important for landing your debuffs on difficult NMs, but theres more than just those items for it. Finally, if you're going to use you inventory space holding this, I seriously reccommend getting it HQ'd via limbus, as the HQ makes an exellent eva peice (+8Eva +8AGI).
This, along with the Ninjutsu Torque, and the Ninjutsu Earring are an invaluable trio when it comes to sticking NIN debuffs on any mob. +15 Ninjutsu skill will make all of your ninjutsu actions as if they were 5 levels above what they are. I have repeatedly stuck debuffs on Kirin and all other gods/HNMs while using these 3 items. HIGHLY recommended.
@ Dubont: Are you serious? You don't play ninja much do you?