I think the idea behind weapon classes is this:
Sword: Quick, sharp and versitile.
Axe: Slow, heavy and sharp.
Mace: Blunt, average speed and stunning (in that concussion sort of way). Bonus vs. undead.
Staff: Very quick, light and often magical. Common with Wizards and Monks.
Polearm: Very slow, very long and very powerful.
Spear: Melee or ranged, powerful, difficult to use.
Dagger: Small, versatile, and sometimes magical. Favourite of backstabbers.
Bow: Ranged, quick and random.
Crossbow: ranged, slow and powerful.
I'm sure I've missed something.
While each has it's benefits over any other, by specialising in one weapon makes for a more interesting game. Are you going to be a slayer of the undead, a all-round warrior, a wizard, etc.? It all comes down to individual play style and help to create the character you play.
Unfortunately, in Divine Divinity, the game strongly urges players towards Swords or Daggers, making my Axe fetish a little redundant.
