Okay upon further investigation, people on the PU ladder seem to have a bit of an unhealth obsession with a... certain item.
Your Guide to Assault Vest in PU
Since soon after its introduction, Assault Vest is an item that has developed a bit of a reputation for being an item that newer players tend to slap on everything without much thought, when in reality it is actually only good on a select few Pokemon. Similar to Weakness Policy in higher tiers, PU is the tier with the fewest Pokemon that can actually make good use of Assault Vest, and have a reason to use it over another item: zero. For one reason or another, every Pokemon in PU is better off running another item than Assault Vest, yet it still sees significant usage on quite a few Pokemon. Let's go through some of the most common ones (this is just the 1630 stats; the 1500 stats are so bad that they're painful to look at) and why they shouldn't EVER be used:
Throh (Assault Vest 24.799%)
This is probably the least heinous of the list, but it's still pretty bad. While it does have a good physical movepool, running 4 attacking moves on it is a huge waste, as it only ever needs Knock Off and Storm Throw, while Ice Punch and Thunderpunch are niche options to hit specific targets. Not only does Throh depend on its Leftovers to not be worn down incredibly quickly, but its non-attacking options, especially Rest + Sleep Talk but also Taunt and Toxic are just way more valuable than more Special Defense, especially since that extra Special Defense doesn't make you that much bulkier because you get worn down so easily. While AV Throh isn't the worst set you could run, it's just limiting Throh's full potential for minimal benefit.
Armaldo (Assault Vest 19.036%)
This is less a case of "why shouldn't you use AV on this" and more of "why would you ever use AV on this". Like Throh, Rapid Spin Armaldo relies on its Leftovers to not be worn down so horribly quickly by Stealth Rock, as Armaldo's #1 priority is to remove it. Armaldo also relies on Toxic to not be a free switch to Poliwrath, but the main reason not to use AV on it is that there's just no reason to. Armaldo is not bulky, so it doesn't make sense to trade in an item that takes away much-needed longevity in exchange for pointless bulk. If you want to give up your Lefties on Armaldo, at least use Lum Berry, which lets you spin on Jumpluff and be better off against defensive Misdreavus. (by the way, this whole paragraph is talking about offensive Armaldo, defensive Armaldo is terrible and should never be used)
Ninetales (Assault Vest 11.820%)
Why would you ever take the best special sweeper in the tier and slap on an item that prevents it from being able to use Nasty Plot? It just doesn't make sense, and Ninetales's other viable sets rely on support moves even more, mainly Will-O-Wisp and Pain Split. This one is geniunely very puzzling to me, with most of this list I can at least see where people would be coming from, but i genuinely have no idea why you would ever use this.
Beheeyem (Assault Vest 17.456%)
Beheeyem's main niche in PU is to kill everything with ridiculously powerful Specs-boosted Analytic Psychics. Beheeyem's role in PU is not to take special attacks and retaliate with moderately powerful ones, that's just a huge waste of its potent wallbreaking potential. Not being Choice-locked is irrelevant too because everything you would hit with your coverage moves bar Probopass and Grumpig is something you need to hit on the switch to beat anyway, as Sneasel, Pawniard, and Mightyena are all faster than you. Just a waste of potential that has no reason to be used.
Stoutland (Assault Vest 9.392%)
This one is a lot like Beheeyem: Stoutland's niches are to rip through offense by being faster than everything with sand support and breaking through Ghost-types with its powerful CB Scrappy Return. Slapping AV on Stoutland is just throwing away most of its potential for no reason. You can't sweep as easily on sand, which makes you much, much less threatening, and if you really don't want to be Choice-locked just use Life Orb. You can't 2HKO defensive Misdreavus with unboosted Scrappy Return either, which removes the entire reason to use that set. Don't even think of using Intimidate either, it just doesn't work.
Dusknoir (Assault Vest 10.956%)
This, believe it or not, isn't even Dusknoir's worst commonly used set. Defensive Dusknoir is complete garbage for several reasons, and it should probably be covered it a separate post in this thread. Anyway, slapping AV on Dusknoir is just taking CB Dusknoir and making its biggest problem, which is that it's horribly weak even with a Choice Band, and making it just that much bigger of an issue. AV Dusknoir's Earthquake has a chance to not OHKO offensive Probopass from full health, and it can't OHKO Mr. Mime from full health with STAB fully invested Adamant Shadow Sneak, just to give you an idea of how pathetically weak it is. It's not even that tanky either because it gets worn down so easily. Just a terrible set that turns a decent niche mon into an awful one.
Aurorus (Assault Vest 22.857%)
By far Aurorus's best set is a wallbreaker set with Stealth Rock and Life Orb. What makes anyone think "Okay, I want to make my wallbreaker a lot weaker and unable to use the best move in the game which also happens to be its biggest niche, just so I can give the Pokemon with the worst defensive typing in the game a small boost to its Special Defense"? It just doesn't make sense, and the worst part about this is that most people who use AV Aurorus probably use max HP as well, which makes Aurorus almost completely useless against offense and makes it lose to some of the walls it should be trying to beat, like Pelipper.
Grumpig (Assault Vest 12.740%)
Grumpig relies a lot on its support movepool and passive recovery from Leftovers. By far Grumpig's most relevant set only runs one attacking move and relies on its unique combination of support moves to be effective. Assault Vest just throws all of that away, this time just to make an already very bulky Pokemon random extra bulk that it doesn't need in this slightest. Like with Ninetales, this one genuinely puzzles me.
Heatmor (Assault Vest 19.063%)
Yet another case of trying to take a frail wallbreaker and make it into a bulky attacker, but just throwing away all of your wallbreaking power and giving you a pointless boost to a stat that your wallbreaker doesn't need more of. Similarly to Aurorus, max HP is also the most common spread on this, which is just dumb and makes Heatmor so much worse at what it's supposed to do.
And finally, the granddaddy of them all, the infamous, the terrifying...
REGICE (Assault Vest 42.900%)
Just... why? Were 100 HP and 200 Special Defense not enough for you? Did you not like how Rock Polish Regice was too good of a sweeper and made your battles against low-ladder opponents too easy and one-sided? Do you love being worn down faster than just about everything in the tier? What about this Pokemon makes you think: "Hey, this Pokemon doesn't have enough special bulk, I have to throw away most of what makes it good in order to get it"? The set that makes Regice so good is one of the least commonly used, which just bothers me severely. If you're going to use this monstrosity, at least use max HP and Special Defense so you can do funny things like take 20% from Life Orb Raichu's Thunderbolt, because funny things like that are about all this set has going for it. I get that having a Pokemon that's almost as bulky as physically defensive Blissey in PU may seem appealing, but AV Regice is horrible for many reasons, and shouldn't get any usage at all, let alone account for almost half of Regice's usage.
Well, that took forever, but it will hopefully let new players see the error in their ways and therefore make the PU ladder a much better place. Assault Vest: together, we can find the cure.
