You know what else gives you Damage Reduction? Armor and Magic Resist; with Health items allowing you to take more punishment (which Health items tend to have armor or magic resist).
And yes, unless your opponent can outplay you, the DUELING item you've purchased will make you much better at winning DUELS. It's not fool-proof nor OP, though. What you gain in dueling power you lose in other areas. Yasuo makes exceptional use of it, due to being an attack speed and crit based champion, but he is still losing out on certain things he could get by buying other items; such as AD and burst magic damage that clears waves (Shiv). Though of course Yasuo will abuse the item, he is an unhealthy champion with basic design flaws (double crit chance is a fun idea in theory, a nightmare to balance around). As for Vayne, the duelist ADC, she loses wave-clear hardcore by not taking shiv. It makes her a better duelist, but she pushes much slower. It's a trade-off. Do you want Shiv to push faster or do you want PD to become an even stronger duelist? I personally say she should stick to her specialty and pick up the PD, but others may have a different idea.
Nonetheless, there are many more champions that don't use the item as well as those two do and they lose a lot if they try to rush it. Tanks, for instance, will often demolish PD rushing duelists, as they can CC them when their jungler ganks (which the DR won't save them from the damage of two people). Also, PD doesn't give AD, which means that auto-attacking and hoping for crits is the best they will do, as their abilities plink off the enemy.
Is PD amazing? On certain champs. Is it OP? Not at all. Is it underpowered then? On certain champs. Does that make it a bad item? Not at all.
Just as with most things in LoL, the game is situational and you have to build according to the situation/where you want your power to go. Everything has its checks and counters, including OP items (which is very rare and I still don't think PD is OP on most champs, rather the champs that can abuse them are who are broken; maybe not OP broken, but broken nonetheless in one way or another) and champs.