WHY DO RANK 2 NUKES PERFORM 15% BETTER BUT COST DOUBLE MP AND TAKE 50% LONGER TO CAST? IT IS A DOWNGRADE IN EVERY WAY IMAGINABLE
Despite the slight contradiction illustrated above (which is excusable, since nothing said in all capital letters on the internet is very articulate), it is apparent that higher-tier spells should have a greater damage boost than they currently have if, indeed, the sole difference between them and tier-I spells is 15% damage. However, if the case is anything like FFXI - and it may be - then the higher-tier spells have both higher damage caps and different resistance equations, making them better against the strongest targets.
For example, Blizzard I in FFXI may have better damage/MP ratio against many general monsters, but it'd be almost useless against high-level monsters, where it would be hitting for double-digit damage only. Something like Blizzard IV, having stronger spell power and/or damage cap, becomes more efficient.