gamefaqs is busted so failing a guide update i'll put some of my findings here.
Basically, I think it's safe to say that one-handed style is avoided by a bunch of players. I've called it a borderline trap build based on how much of a drop in DPS it is compared to dual-wielding or even 2h (which also is a drop from dual-wielding but makes up for it with extra PEN and/or reach). Basically it only existed in my mind as a setup that desperately wants crits on PotD (where even the much higher number of attacks from 2w might not make up for the fact that you have 0% chance to crit, if you rely on those crits), but there's not really that many builds that care *that* much about crits.
But one place where single-weapon style starts competing well with dual-weapon style is on characters that are already very fast. Yes, Action Speed is Linear Returns. But if you have multiple dimensions that feed into damage and a fixed amount you can put into those dimension, you maximize your outcome by balancing those dimensions (the area of a square is larger than the area of a rectangle, if their perimeters are equal). So for very fast characters, you start doing better if you make each of those weapon attacks ''better'' instead of making yourself even faster by dual-wielding.
Keep in mind you have to be very fast or effectively very fast for the trade-off to make sense - you just get so much action speed for "free" by dual-wielding. It takes a lot for the single-weapon case to be noticable versus the consistent returns on increasing action speed. You need high dexterity (20+), with action bonuses ('''Deleterious Alacrity of Motion''', '''Frenzy''', '''Swift Strikes''', ''Cat Flurry', or consumables), with minimal armor encumbrance (at most comparable to light armor, even better if it's a piece of light armor with a recovery time bonus, cloth, or light armor with a pet or passive that reduces armor recovery penalty). On top of that, it helps to be a swashbuckler with a -50% recovery time bonus, or a monk that gets a bunch of free attacks from '''Swift Flurry''', or some other class that can abnormally boost your effective attack rate--even the barbarian's barbaric retaliation would help. At that point, the fact that you aren't getting as many attacks as a dual-wielder is made up for the fact that your attacks are doing more damage overall and critting more often.
Aside from this, even with "conventionally" fast characters, single-weapon style is great for when you're expected to underpenetrate the enemy. In such a situation, even a more normal single-weapon user can overperform a dual-wielder who has to flip on a weapon modal that grants +2 PEN for +50% recovery time penalty. This is all because of the fact that crits get a 1.5x multiplier to your PEN, so getting more crits from both +12 accuracy and a hit->crit rate is a huge win over time, even if on any given individual hit it seems like the +2 PEN on a dual-wielder with a weapon modal would do better than a single-weapon attacker without a weapon modal. For a martial character with spare weapon slots (like a Blackjacket or someone equipped with Fleshmender) it might even be more worth investing a point into single-weapon style and reserving one weapon slot for single-weapon-wielding, even if that's not your main focus, for precisely these PEN-related reasons. This PEN effect also has the implication that if you like using weapons like daggers, flails, and hatchets, which don't have a +2 PEN modal, you may want to consider a partial single-weapon style setup over pure dual-wielding setup for precisely the underpenetration cases. (This is much more of an issue on Veteran/PotD than on lower difficulties)
I ended up making single-weapon style work pretty decently on a SC barbarian wearing basically cloth (actually, the changeling's mantel with the -10% recovery time bonus and Nalvi) and who would get a Quick buff from a chanter. Aside from the calculations, it definitely "felt" ok, too, and it was a huge difference between cloth/fast light armor and even medium armor - with normal medium armor it still "felt" bad compared to dual-wielding. (Though in the end, spamming Driving Roar is what really ended up mattering )