So I see a lot of people saying that King's Shield 50/50s are fun because they make the players think, or they make the player with the better prediction skills win, or whatever.
Anyway, they are not skill based.
I'll go to a scenario that I've mentioned before briefly, but back in like page 11 or whatever. It's a bit artificial to best show my points, but it is very similar to the situation that most contact attackers are in.
I have a Life orb Weavile with Swords Dance and Knock Off, my last excuse for an Aegislash check remaining. You're down to your Aegislash, and I sacrificed a mon to let Weavile come in on Blade Form.
If Aegislash uses King's Shield and Weavile uses Swords Dance, or if Aegislash uses Sacred Sword and Weavile uses Knock Off, I win (well, it could use a second King's Shield, but no one's disagreeing that's a luck based 50/50). If Aegislash uses KS as Weavile attacks, or Sacred Sword as it sets up, I lose. The reason I picked Weavile vs. Aegislash is because I can prove that the risk and reward for both moves are equal, either Weavile wins or Aegislash does depending on how their moves match. While this isn't the case for most pokemon, King's Shield has pretty similar risk and reward especially on contact attackers.
This post by Jukain explains it pretty well, that you are not making an educated guess based on what type of situation you and the opponent are in, you are simply guessing whether Aegislash will use King's Shield or keep attacking.
There's a reason why we call these situations 50/50s. Because the mathematically optimal way for both players to play them is to literally flip a coin to make their decision.
First, your choice should be random, because if you have any kind of patterns, the opponent can exploit them, so even in other situations where you're heavily weighted towards one option or the other, it's still best to not make patterns out of it that the opponent can guess.
Second,
when the risk and reward for both of your moves are equal, you don't want to favor one option over the other. If I instead have a 60% chance of using Knock Off, and a 40% chance of using Swords Dance, the Aegislash will just hit King's Shield and win the matchup 60% of the time. It's best to split the odds of both moves 50/50 so the Aegislash user only wins half the time no matter how he makes his choice.
Of course there's that big bolded part. I believe it is true with contact attackers. Let's go through the possible scenarios, using instead something like Tyranitar without Earthquake.
Tyranitar uses Crunch, Aegislash uses King's Shield: Loss in momentum for Tyranitar, a free turn for Aegislash as Tyranitar is forced to switch or do pitiful damage.
Tyranitar uses Crunch, Aegislash attacks: Aegislash takes huge damage, possibly KO'd.
Tyranitar uses Stone Edge/Stealth Rock/etc, Aegislash uses King's Shield: A free turn for Tyranitar. Hope it has something better to do in that turn than Stone Edge to avoid the attack drop.
Tyranitar doesn't use Crunch, Aegislash attacks: Tyranitar takes massive damage, possible KO'd.
So the risk/reward are fairly equal: Either T-tar or Aeg takes huge damage, or one of them gets a free turn. This is a true 50/50 (or 55/45 or something very close).
The fact that King's Shield can force Tyranitar not to use Crunch when it's super effective because of these 50/50s is just stupid. Get Aegi outta here, or at least King's Shield (can someone please confirm/deny this as an option? sorry if I missed it but I read this whole thread)
