Gonna post this from yesterday (for some reason I couldn't) 'cos it still applies:
Sorry to cherry pick, but I just wanted to emphasise this point a little, as I feel like people are not taking this argument seriously and are discounting it too easily when it is a real issue.
As Rick says, Aegislash forces excessive 50/50's, more so than Bisharp's sucker punch. This is also due to the sheer versatility of sets that it can run effectively - Bisharp will always have Sucker Punch for example, but Aegi can function very well without KS, and the opponent always has to take the risk. If Aegi does have KS, it's a real 50/50, but most importantly, it's ALWAYS in Aegi's favor, due to the attack reducing effect.
Knock off is a really common move in the meta now, and its definitely tempting to use against Aegi; the problem is, if you don't have Bisharp, you now have to risk ending up with -2 Atk and losing a huge chunk of health.
-2 252+ Atk Conkeldurr Knock Off (97.5 BP) vs. 252 HP / 0 Def Aegislash-Shield: 88-104 (27.1 - 32%) -- 39.6% chance to 4HKO after Leftovers recovery
-2 252+ Atk Huge Power Azumarill Knock Off (97.5 BP) vs. 252 HP / 0 Def Aegislash-Shield: 90-108 (27.7 - 33.3%) -- 88.4% chance to 4HKO after Leftovers recovery
-2 252+ Atk Choice Band Technician Scizor Pursuit vs. 252 HP / 0 Def Aegislash-Shield: 78-92 (24 - 28.3%) -- possible 5HKO after Leftovers recovery
-2 252+ Atk Choice Band Scizor Pursuit vs. 252 HP / 0 Def Aegislash-Shield: 102-122 (31.4 - 37.6%) -- 0.1% chance to 3HKO after Leftovers recovery
All of a sudden, you lose the momentum while not trading that much damage. Granted, it's not something that Aegi can do repeatedly, but it doesn't need to. It's not going to sweep, it wants to pull apart teams, not to finish them off (apart from sets like SD, Autonomize, and WP sets). It's just so difficult to stop it doing its job.
Another point made earlier is that KS also makes Choice items a liability to a certain extent, especially on balance and fatter teams. KS, with it's -2 effect, is what pushes me to believe that Aegi shouldn't be in OU, especially when combined with its power, bulk, and versatility.
Without KS it wouldn't be anywhere near as good, as it would have to use protect on defensive sets and lose out being able to make the most of its ability - it would just be used as an offensive hit and runner.
It doesn't even need to be stated that while 50/50s are present in the game anyway, Aegislash forces excessive 50/50s, and it isn't too uncommon for games to be decided by whether the opponent uses KS or not. Hopefully, suspect voters can agree that a metagame where 50/50s are not overly excessive is preferable to a metagame where 50/50s are, and see that this is yet another reason to keep Aegislash in Ubers. As for the KS ban, Aegislash has sets, mainly the All out attacker one, that are still very effective without it. If it doesn't contribute to the meta by alleviating matchup issues, and dramatically limits options in building, there is still no reason to bring it down.
Sorry to cherry pick, but I just wanted to emphasise this point a little, as I feel like people are not taking this argument seriously and are discounting it too easily when it is a real issue.
As Rick says, Aegislash forces excessive 50/50's, more so than Bisharp's sucker punch. This is also due to the sheer versatility of sets that it can run effectively - Bisharp will always have Sucker Punch for example, but Aegi can function very well without KS, and the opponent always has to take the risk. If Aegi does have KS, it's a real 50/50, but most importantly, it's ALWAYS in Aegi's favor, due to the attack reducing effect.
Knock off is a really common move in the meta now, and its definitely tempting to use against Aegi; the problem is, if you don't have Bisharp, you now have to risk ending up with -2 Atk and losing a huge chunk of health.
-2 252+ Atk Conkeldurr Knock Off (97.5 BP) vs. 252 HP / 0 Def Aegislash-Shield: 88-104 (27.1 - 32%) -- 39.6% chance to 4HKO after Leftovers recovery
-2 252+ Atk Huge Power Azumarill Knock Off (97.5 BP) vs. 252 HP / 0 Def Aegislash-Shield: 90-108 (27.7 - 33.3%) -- 88.4% chance to 4HKO after Leftovers recovery
-2 252+ Atk Choice Band Technician Scizor Pursuit vs. 252 HP / 0 Def Aegislash-Shield: 78-92 (24 - 28.3%) -- possible 5HKO after Leftovers recovery
-2 252+ Atk Choice Band Scizor Pursuit vs. 252 HP / 0 Def Aegislash-Shield: 102-122 (31.4 - 37.6%) -- 0.1% chance to 3HKO after Leftovers recovery
All of a sudden, you lose the momentum while not trading that much damage. Granted, it's not something that Aegi can do repeatedly, but it doesn't need to. It's not going to sweep, it wants to pull apart teams, not to finish them off (apart from sets like SD, Autonomize, and WP sets). It's just so difficult to stop it doing its job.
Another point made earlier is that KS also makes Choice items a liability to a certain extent, especially on balance and fatter teams. KS, with it's -2 effect, is what pushes me to believe that Aegi shouldn't be in OU, especially when combined with its power, bulk, and versatility.
Without KS it wouldn't be anywhere near as good, as it would have to use protect on defensive sets and lose out being able to make the most of its ability - it would just be used as an offensive hit and runner.