Forgive my ignorance... but why is that the case with FONV and not FO3? Did the damage formula change or something? I always thought a higher DT was good...
Fallout New Vegas uses damage threshold, Fallout 3 uses damage resistance.
Damage threshold is a set number that you subtract from the incoming damage, or if your damage threshold is above the incoming damage, you simply recieve 20% of the incoming damage instead. So if I have a damage threshold of 25, then a bullet that does 20 damage will instead only do 4. If I have a damage threshold of 20 and a bullet does 40 damage, I only take 20 damage.
Damage resistance resists a %. So if you have 50 Damage resistance, it reduces all incoming damage by half. By the end of FO3, you could easily obtain a Damage resistance of 85% with the help of Power Armor.
The reason people say Power Armor svcks in New Vegas is because Light armor and Medium armor are both capable of giving you a damage threshold of 30, whereas Power armor may give you one of 40 or 50. There are very few guns (or few enemies with said guns) that hit between 30-50 damage per shot, so LA or Medium armor will effectively give you the same benefit as Power armor 90% of the time. Plus, let's say a gun hits 45 damage and with power armor, your damage resistance is 50. This means that instead, you will take about ~10 damage per shot. The LA or MA equipped player, on the other hand? He'd take 15 damage from such a gun. Or let's take an AMR for example. An AMR deals 110 damage. If you get shot by one with LA or MA, you'll take about 80 damage, whereas with power armor, you take 60 damage. Again, not a huge difference, and quite pathetic when you consider that, best case scenario, your power armor is protecting you from 20 more damage. To top it all off, it's heavy. About 45 pounds of weight; even with the max carrying weight, that's about a 6thed of your carrying weight.
At the same time, Light armor can provide other benefits via perks. It provides increased mobility to help you dodge in fights or run away, increases your crit rate by 5% and also does a fairly nice job of preventing enemies from critting you. Considering that a crit from an AMR on very hard mode could mean death for almost any character or armor setup, Light armor may be more beneficial. And the weight issue? Light Armor is often a mere 8-20 pounds of weight. Thus, Light armor is the best armor.