Kerrick said:
Maybe it's just me, then, but I get the image of the archer in the tree (we'll use this for an example) shooting arrows into combat (an ambush, if you will). He's shooting targets, made his attacks for the round, and he spots someone casting a spell. He should be able to draw an arrow and shoot at the guy. You wouldn't be able to do it with, say, a crossbow, because it requires a move action to load, but a bow, dagger, shuriken... why not?
Bolding mine.
Kerrick, you are of course free to make whatever house rules you like in your own game. If you are genuinely asking for reasons why a DM might not allow this, I'll give you mine:
When you threaten enemies in melee, they are necessarily close to you. In most cases, they are within 5 feet of you unless you have a reach weapon. This makes it relatively easy to keep track of what they are doing and whether they take an action that takes their mind off of defending themselves.
An archer (or somebody with a dagger or shuriken) standing 30 feet away from the a pitched melee is not necessarily able to to pay as much attention to what everyone is doing. They are further away from him and with the chaos of combat going on all around him, he simply can't make a quick enough determination of when someone has let their guard down. Unless of course he is specifically watching a particular enemy Wizard getting ready to cast a spell...
Which is precisely where Readied Actions pick up.
The other exceptions to this are certain prestige classes that allow you to threaten an area with your bow. This is the result of having gained such incredible mastery of the bow and a level of comfort being in the middle of combat without benefit of a melee weapon that you are able to react to someone letting their guard down with lightning speed in order to get a shot off.
To me, this reasoning is fine but I still maintain that if you don't like it, you should absolutely modify the rules to your tastes (assuming that you are the one running the game). I will say that if one of the DM's that I play under introduced this as a house rule, I'd have several reactions based on it:
1) I would probably be hesitant to play a spellcaster, especially a Wizard or Sorcerer.
2) I would probably be inclined to play an archer since one of the major drawbacks (and, I would argue, balancing factors) had been removed.
3) I would strongly urge the rest of the party to attack enemy archers before anybody else, especially if we actually had a Wizard or Sorcerer in our group.
Add to that the fact that rules about AoO's become very inconsistent (i.e. do you provoke AoO's when moving in the Archer's "threatened area"?) and I'd be reluctant to play in such a campaign.
YMMV