The theory behind it was to simulate the most likely feats that a light-weapon wielding Fighter or Rogue would take - Weapon Finesse and Two-Weapon Fighting. I wanted to reflect a Fighter's proficiency and a Rogue's skill with light weapons. As it stands, a Mage or Cleric can use any weapon at all without penalty - meaning sword-wielding Magi don't need to multi-class, so no multi-class rules were needed in Core, and a Cleric can use the Chosen weapon of their God without penalty. I figured giving them more of a buff in combat than that was pushing it.
Outside an Oriental-style setting where I'd judge anyone can wield weapons specifically designed as a weapon pair (tonfa, sai, etc), I'd be wary of giving those benefits to spell-slingers as their power-level plays just fine as it is.
In combat, Fighters are the best thanks to their +1 attack and damage (simulating any number of feats a D&D Fighter would take), then Rogues as they can also use two-weapons and benefit form their typically high DEX with light weapons. Next up is Clerics because of their ability to use armour, then Magi - though a Magi in Mage Armour wielding a Greatsword is a sight to behold!
I have a player in my game who's playing a Magi with a Greatsword who's running him as a Pictish Warrior. He's covered in Woad, almost naked and calls spirits from the campfire, effigies around his neck and the earth itself to aid him in combat. Scary, but brilliant.
All characters get multiple attacks after their total attack bonus reaches +6, just taking a -5 on each additional attack after the first. If you really, really wanted a two-weapon wielding Mage there's no reason why you couldn't apportion those additional attacks between a weapon in each hand. So a Magi wielding paired shortswords could hit with the first at +7 and the other one at +2, say. At lower levels they just don't have the training, though theoretically a STR 18 Magi could manage two attacks at +6/+1 by level 2.
Make sense?
In contrast, a 2nd level Fighter with STR 18 could use those shortswords at +7/+2 (thanks to their +1), or +5/+5 because of their Two-Weapon Fighting training.
Of course, I'm open to be convinced, but feel that making TWF and Light weapon use open to anyone, you're weakening the Fighter and Rogue rather than improving the game..... but what do I know?
