Vitality and Spirit do have an additional effect: Vitality increases your Energy gained during battle (see Bonus Energy on the status screen) while Spirit also reduces MP costs.
Subtlety, Bushido, Plunder, Darkness, and Chi skills are all based on Strength for damage. However, Xelltath's Dark, Darkra, and Darkja skills as well as many of Rei's skills also gain a additive damage bonus based on Magic. All magic spells for Phoebe, Lena, and Malus are based on Magic for damage (see the Compendium for the exact formulas if you're curious).
The original idea behind the Berserker was that it made the character Auto-Berserk. The berserk state increases damage and speed but you lose control of the character and cannot use their special attacks. I changed it to its current iteration in order to maintain that same spirit but at least give you control of the character; hence, it increases damage while preventing the use of special attacks by making you lose energy. However, the energy penalty can be offset by by increasing Vitality, equipping relics that increase energy gain, being in the middle row, and using an Energy Mode that grants a lot of energy. For instance, use the Fighter energy mode and then equip a Genji Glove; your two attacks per turn will generate 6 energy from that alone.
Speaking of the Genji Glove, keep in mind that it reduces the damage of both attacks by 75%, but you're getting two separate attacks. It is always a damage increase, but the closer in strength that your main and off hand weapons are, the better. For instance, let's say you can make 1 attack that deals 100 damage. With a Genji Glove, you would be making two attacks at 75 damage apiece, for a total of 150; a 50% increase overall. The damage increase you get from a Genji Glove depends on which weapons you use: to get the maximum, it's best to equip the same weapon in both hands (like 2 Daggers or 2 Bronze Swords).
Also keep in mind that the Genji Glove does not affect magic. It's a perfectly viable tactic to have someone like Phoebe dual-wielding rods because the magic bonuses from both weapons will stack and are unaffected by the Genji Glove's damage reduction. And again, having two separate attacks will give you much more energy from any energy mode that generates energy from dealing damage.
Ultimately, it's up to you and if you feel the sacrifice of a shield and relic slot to dual-wield isn't worth it, that's perfectly fine. The game is designed (and this was the most time-consuming part balance-wise) so that there is no "wrong" way to play. There are no "bad" character builds or setups, just sub-optimal ones.
As for the error you got, it's already been fixed, just another audio file that was missing. If you have an older version of the game, you can just copy Wounded.ogg into your current game folder.