So what is the benefit of having someone take a negative to their attack verse you just gaining more AC? With nothing to deny your AC besides shield bonus it seems pointless. Can someone explain it to me?
First level fighter, +3 str, two weapon fighting style.
Would normally hit on a 5+
First level blade dancer, +3 dex, swashbuckling +1 AC, Hide.
Would normally have AC of 7.
The fighter will hit the blade dancer when he rolls a 12+ or 45% of the time.
Cast shimmer to reduce opponets attack throw to 7+, meaning he hits on 14+ or 35% of the time.
Instead let us increase the blade dancers AC by 2, for a total of 9. Which means the fighter hits on a 14+, or 35% of the time.
Seems pointless in the long run and adds more math to get wrong.
It is true that there is no mechanical difference between +2 AC and -2 to hit for the opponent.
I don’t see how it really adds more math to get wrong, given the core ACKS mechanic being that you apply your AC as a penalty anyway. But if you prefer to phrase it as +AC, there’s definitely nothing stopping you.
It's a legacy effect from the game's OSR roots.
In the original B/X, Displacer Beasts imposed a -2 penalty to hit on attackers due to their displacement field. Shimmer was created to originally intended to be the spell that a caster could use to replicate Cloaks of Displacement, so it was written up the same way.
It is true that there is no mechanical difference between +2 AC and -2 to hit for the opponent. I don't see how it really adds more math to get wrong, given the core ACKS mechanic being that you apply your AC as a penalty anyway. But if you prefer to phrase it as +AC, there's definitely nothing stopping you.
Its a problem I've noticed in larger groups when a lot of different moddifers are being thrown around. Also we do the advantage hits AC. We have 7 people in ours right now and in combat it will go something liket his a lot of times:
"I rolled a 5, so advantage 1"
"Did you count the charge?"
"Don't forget Bless!"
"Ok advantage 4."
The more bonuses/penelties from more places people start to forget or are not paying attetion to. And the stranger the bonus/penelty the easier it is to forget. DM can easily remember my spell gives me +2 AC because thats a normal thing, its just a bit harder to remember that it reduces attacks by 2. Even more so when you are trying to keep track of six other people and the monsters they are fighting.