Weapon Proficiencies (DMG)

Sooner or later a player will complain that the weapon proficiencies are too restrictive. But the real complaint may be that the rules don't allow a character to do everything the player wants.
For example, say a player character is proficient with a long sword. He's about to be overwhelmed by a horde of kobolds, but he has the sense to retreat. Unfortunately, he trips over his feet and falls face-first to the floor! His faithful, trusted long sword skitters from his grip and the little monsters are upon him. Still full of fight, the character wrests a short sword from the nearest beastie and begins to do battle.
At this point, the DM tells the player to apply the nonproficiency penalty. The player howls in outrage. "It's a sword," he moans. "My character can use a long sword, I can't believe you won't let him use a short sword! It's the same thing, just smaller!" Before giving in to the player's protests, consider the differences in what seem to be similar weapons.
The character's customary weapon, the long sword, is a slashing weapon. It is three- to four-feet long, heavy, and balanced toward the blade to increase momentum in a slash. A short sword is a piercing weapon. It is 12 to 18 inches long, light (for a sword), and balanced with most of the weight toward the handle for quick reaction.
So, in our example, the character leaps into the fight using the short sword instinctively—the way he would use a long sword. He tries to slash, but the weapon is too short and light for slashing. He tries to block and parry and finds the weapon absorbs much less impact than his massive long sword. He tends to attack the air, because he is used to the reach and sweep of the long sword. He throws himself off balance by swinging the light weapon too hard. All these minor errors make him less effective with the short sword, even though it seems similar to his long sword. The nonproficiency penalty begins to make sense.
Further, weapon proficiencies are just some of the many factors that must be balanced for a successful adventure. If a variety of factors combine to give a character excessive combat bonuses, the DM should create situations in which that character's favorite weapon is not the best choice.
For example, a character who is proficient with all types of swords, but no other weapons, is at a big disadvantage when confronted by skeletons. His sword is less effective than a mace. Eventually, the player will have to broaden his character's weapon proficiencies if he wants to thrive in the AD&D game world.