So, in theory, yes, it would be cleaner across the whole game to only have one set of effects that deal damage per spell, instead of splitting the spell at the effect level into two effects, one that's applied to monsters and the other that's applied to players. In practice, however, these changes aren't made in a vacuum. To adjust the spell power of every monster in the game to make the old spell match the new spell is not a trivial change, and it would, well, adjust the spell power of every monster in the game. There's a huge gulf between "the theoretical most elegant solution" and "the solution that works" in this case. Adjusting the spell power of the monsters that cast Melf's - even just the Acid Spell Power - would mean that any other Acid spells they cast would be weakened - something that we don't really want to happen. To that end, the best way to fix this bug is the way we have opted to proceed - adjust the malfunctioning spell directly to limit unintentional changes.