The way I see it is that this advice is mostly for players who have reached the point in their progression where they start thinking about doing some of the harder missions. It is true that for most of the game they will be just fine with a Chaos armor set, but I can imagine that when that player sets out to do Compound 42 for the first time, they will be poisoned and set on fire constantly and die a lot. Then it can be useful to have a more resistant armor set. Say they use mostly Guns and have a Shadowsun set, they'll have good gun damage and resist the poison quite well. And then they might have a Gran Faust with Gremlin High they can use to clear some space.

Do you have a link to the player research regarding Armor damage UVs? I certainly notice a difference when using proper defensive armor with matching defense UVs.

Thanks for going through the trouble finding those threads. I already read some of those in the past. That research is indeed useful, but to me it doesn't seem that they prove (or tried to prove) that defense UVs on armor are useless. In my experience, it is not that I find myself surviving an additional hit and then die, rather it is that I find myself able to preserve my hp for much longer.

Consider the following points:

* A Skolver set with 2x piercing max will have 177 piercing defense (on depth 25), correct? A Black Kat set or Chaos set would have no piercing defense. If you then get hit by 360 points of piercing damage wearing those, you'd take the full 9 bars of damage. But the 2xMax piercing Skolver set would take much less than that, around 4.5 bars of damage. A standard no UV skolver set would take about 6 bars of damage.

* Knights have a very large health pool in PvE. It's true that if you have 27 pips of health and you take 3x360 damage, you are dead (let's ignore sonic mode). But a Knight has much more health than that: Hearts that restore 1-3 pips drop frequently, Pills will restore 12 pips. There's also the Emergency revive giving you up to 30 extra pips once every level, should you need them. Taking 4.5 bars of damage as opposed to 6 bars of damage (2xPierce Max UV Skolver vs Standard Skolver) when you have 30 pips only let's you survive 2 additional hits. But if you consider 30 pips + 30 pips from the ER + 3 health pills (36 pips) + 8 big hearts (24 pips) you actually have about 120 pips of health before you need to use a spark of life. So over the course of that depth, a standard skolver will run out of health after about 20 hits of 360 piercing damage, while the Max/Max Skolver will still be alive after 26 hits. A knight wearing chaos armor or black kat would be on their last bit of hp after 13 hits. Ofcourse this is just theory, and it is likely that in a real level you will also sometimes take an elemental or shadow hit even if the level is primarily piercing themed, which will not work to Skolvers advantage.

* The higher the damage numbers get, the less impressive a flat reduction of 26 or 52 points looks. So if a monster hits you not for 320 or 360, but for 480 base damage (I don't know if this actually happens often), then the value of that additional 26 or 52 may diminish a bit. However, poison is one way to help reduce monster damage to keep it closer to the normal range. Also, the biggest hits are usually easier to avoid and it is often a stray bullet with relatively low damage that ends up hitting you.

To conclude: In my opinion, damage UVs when they match your armors inherent defense and are +3 or +4 are useful (for example Shadow max on Black Kat is a matching UV). Their value depends on the size of the hit you actually take. They will have more effect against multiple small hits as opposed to 1 large hit (since they are a flat reduction). In my experience the largest monster hits are usually easier to dodge and smaller hits are more likely to be what hits the player, which is in the UV's favor. Their value also increases with a higher total health pool including healing sources. Using Poison weapons can scale down large hits, further improving the value of defense UVs.

Non-matching Max defense UVs might be useful (e.g. Elemental Max on Black Kat) but this depends entirely on how large monster hits usually are. Though it is certainly better to have 2x Elemental Max on your Black Kat set, than 1x Piercing Max and 1x Elemental Max.

The 125 piercing defense on Skolver is for the set, right? 62.5 piercing defense per piece, its normal defense sits at 71 per piece or 142 for the set. 1 max UV is 26 points, so 2 max UVs will add 52 points. 125+52 points is how I got to the 177.

The first time I used Chaos Set, I was solo on a freeze level and I ended up getting locked in a freeze chain. The first hit that froze me essentially killed me as well. Though this inevitable death spiral is mostly dangerous when playing solo. A party can make up for it.
Shock is bad but the dash can usually get you out of harms way and with a strong shield you can tank 1 or 2 attacks before the shock wears off. Again having a party makes it a lot easier because they will grab some of the aggro, especially after you dash.
That said I do also protect against statuses first and the armors that I use primarily (Skolver, Snarbolax, Black Kat) all have inherent freeze resist and I have shock resistance UVs on them.
But nonetheless, I am a lot more tanky on shadow and piercing levels than on elemental levels (although on elemental levels I equip grey owlite, heater or dragon scale shield to make up for it). Especially on piercing levels I always feel like a tank when I have my Skolver with 2x +4 equipped. Sometimes I want to use guns on piercing levels (like my Mega Tundrus against Giant Lichen Colonies) so I equip Black Kat, and then I also take 8 bars of damage from the average monster attack.

So I also value status UVs higher than defense UVs, even for party play where teammates will help prevent 1 status effect from ending in a death spiral. That said, +3 or +4 defense UVs are still far from useless in my opinion.