I'm a diehard dual-wielder, no question about that. When you factor in the +1 attack ability of Belm, Kundane, and the Scarlet Ninja-To (for bards and thieves with UAI) and Improved Haste, the power is just too great to ignore.
I think all the styles have their uses except Sword and Shield style, because it only grants an AC bonus vs missles. If it were an all-round AC bonus, I'd use it occasionally on characters like Anomen, but as is I'm not a fan of it. (That being said, I understand some mods create some pretty difficult missle attack encounters where it might come in more handy.)
I'll usually give Minsc and Keldorn two-handed weapon style, and occasionally I will give my Fighter NPCs both two-weapon style and two-handed weapon style if they have enough proficiencies available. (Extra chance for criticals with Staff of the Ram or Carsomyr? I'll take that.) Since I rarely go beyond specialization in any weapon, that is often the case for single-class or dual-class Fighters. Multiclass fighters, however, have too few proficiencies to focus on more than one style.
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