From field experience with melee weapons and thrown weapons.
Firing a missile weapon or throwing a thrown weapon requires a given stance that prevents any defensive action.
When facing a melee weapon, if you do not defend yourself actively, you are hit. Period. It takes half a second for a novice fighter to deal a blow that will give him the time to ready a killer (a slap from the flat of a sword blade on the arm is enough). In the same time a trained fighter would have dealt a killer, an expert would be jumping over your cloven body. Not defending in melee is not an option.
If you want to disregard this, then the physics of projectiles come into play. Most thrown weapons rotate. Right after they are let free, they can’t do much damage (the target is truck by the handle) except for shurikens and some african throwing irons. Weapons like throwing spears, javelins and the good old fashioned rock are not rotating, darts too. However, they are lobbed and actually deal more damage when falling. At close range, most thrown weapons are either inefficient or lacking.
Missile weapons are another waltz. Crossbows could be of some use, but a knock can unsettle the quarel (and believe me, knocked you will be), arrows shot from some bows are swinging crazy before flying straight (powerful bows tend to send the arrows straight, less powerfull ones are another story, I’ve seen an arrow do a 360 after the string was released and then resume its flight, but I guess the D100 covers this kind of event). Other weapons all have their flaws. Basically, weapons designed to hit targets away are not that great to hit targets in melee.
So, while not impossible, I’d say they are, in our world, quite unsound fighting methods. People arming a throw or bracing a missile weapon are just sitting ducks in front of a guy with a meat cleaver.
I decided to give a +100 OB bonus to anyone meleeing someone that does not defend himself, on top of removing any quickness bonus BD. Since firing a missile or throwing a weapon prevents from defending yourself, you get the picture.