6) The +X on a magic sword represents the amount of positive magical energy in that weapon. When you meet a AC -3 creature, you need to make your THAC0 roll and you must have a +3 weapon because you need that much "positive" magic to counter the "negative" magic that protects that creature - thereby allowing your physical weapon to hurt things like ghosts and stuff. Part of going into dungeons was to retreive these things and most monsters would not weild magic items because they were "positive" magic.
I really like this.
There is a point in the campaign, were everyone has a magical weapon. So the monster ability becomes pointless, or greatly reduced in power or feel of a powerful monster.
With this rule, monsters hit only by magic, once you have the weapon, will still have an extremely hard to hit AC. The maximum AC would be -3, reserver for powerful demon lords and gods.
With this rule, at high level play, the monster ability to be hit by magic only will still be meaningful.
The rule is also great for stat blocks. If the Statblock says AC -1, you automatically know the creature is hit only by +1 weapons.