I know what they aimed at, but the effects are just not worth it.
you would be better overall to take +1 AC and be done with it.
it's even more absurd with using both dueling and GWF with a Versatile weapons.
IE; Longsword,
1d8+5, 60% hit, 5% crit
dueling, 1d8+7;
6,9+0,225(crit)=7,125 per attack
GWF, 1d10+5(treat 1&2 as 3)
6,48+0,29(crit)=6,77 per attack
so anyone that is having both Dueling and GWF and using a longsword(for whatever reasons would you make that character) for it, is better at using it 1Handed than 2Handed.
yes, you are better with damage die riders, but base weapon damage should be better for 2Handed use than 1Handed if you have both styles and the weapon can utilize both styles.