It's more interesting than just a buff/debuff concept. A buff works on any move that matches that general/offense type, but this would be using two offensive moves in order to achieve multiple possibilities: It could be more damage, or special effects. For instance fire jutsu -> wind jutsu = amplified flames. But if you use fire -> lightning, you gain nothing. Water -> lightning, however, might add a stun effect. etc etc etc, millions of possibilities, fits the themes of the series, seems like a good idea.