Two weapon style is worth it if you're looking to do maximum damage. My characters on two weapon style at the endgame had 2K plus damage (max intelligence and strength including upgrades). Summing up all the bonuses I think my last character reached 3K damage on DKS.
In all my playthroughs, two weapon always beat the two handed and single weapon on damage. I didn't like this cause I wanted to go two handed (Like you I wanted to emulate a D&D character specifically a power critical, epic weapon focus/specialization greatsword barbarian)
It's been a long time since I played on normal difficulty but IIRC there's a point where the player's ideal build will take off. When it does don't be surprised if the game becomes too easy up to the end.
Now, all my playthrough on nightmare I'll have to confess, I went through broken valley mostly by
separate them from the pack and plink away at them one by one.
When I got to level 18+ with good armor and decent damage, that's the only time I started to go toe to toe with the enemy. BTW, the summon ghost skill is a time/potion saver since it will heal you.