I like this discussion, because one of the very first things I house-ruled when I started DMing 3rd edition (this is 3.0 mind you) was to add 'bludgeoning' as damage type to several of the larger melee weapons.

I had always been told that you can batter an opponent with a large weapon like a greatsword, even if his armor is protecting him from slashing-type weapons. This is especially evident against maille type armors - they protect from cuts, but a solid blow from any heavy weapon can still break ribs or elbows or knees.

Frankly I don't think the balance issue is important. Yeah, sure, now the PCs are less likely to wind up facing skeletons or whatever and realize they lack effective damage-type weapons. But my players have always been smart enough to ensure the party has ample supply of all three damage types anyway.

Of course, I never really cared much for the three damage types, either. To me, there should be a fourth - chopping - becuase you generally can slash OR chop with a number of edged weapons, and the results (and type of armor that resists the damage) vary. Plus, of course, there is a difference between the piercing damage of a stiletto and the piercing damage of a broadhead arrow. One is designed specifically to penetrate maille and puncture light plate, while the other is designed to create a very bloody wound in an animal, or be difficult and damaging to extract from a human.

Ultimately I tend to view these things as being guidelines. I freely apply minor advantage or penalty to various weapons against specific foes, based on flavor considerations. I think this helps prevent weapon-optimization arguments at the gaming table, and encourages the players to pick weapons that fit their character concept, rather than some calculus about the types of enemies to be encountered.

-Lep