The Dev team actually touched on this a while back, I can see if I can find the post, but the short of it is that the Spell names were done to not confuse new players to the series. In Japanese the spells follow the normal convention of no suffix, Ra, Sa, Ga, Ja, etc, but because not every spell is the same in that some are AoEs and some Single target, they thought it might get confusing for people not familiar with the series as a whole, which is why they did the I, II, III, IV method. But regardless of which method is used both are meh for the spell hierarchy created in this game.