Point of Shimmer and similar spells?

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?

 

Example:

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.

 

[quote="Aryxymaraki"] 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. [/quote]

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"
"Miss."
"Did you count the charge?"
"Don't forget Bless!"
"Ok advantage 4." 
"Hit."

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.