The method you're proposing sounds nice in theory but in practice it doesn't work. Any smart player would figure out what the monsters were doing and realize they could do it too.
Player X. I ready an action to fireball the room after Player A kicks the door in.
Player Y. I ready an attack at any monsters who survive the fireball.
Player Z. I ready an attack against any creatures who survive the archer's attack, partial charging if necessary to reach them.
Player A. I kick in the door and take a 5' step.
DM. OK, so the monsters had readied actions too. [Spends several minutes comparing initiative modifiers and rolling to break ties] So, Player X you have the best init modifier so your action goes off first.
Fireball.
DM. And Player Y also has a better init modifier than the monsters so he acts next.
Fires his bow.
DM. The monsters have the next best initiative so they take their readied actions: all the orcs throw javalins at the best target. And the ogre charges.
DM. Now player Z, your readied action goes off and you attack the ogre.
This is bad for several reasons.
First, adjudicating a dozen readied actions is a pain--especially when some (like players X, Y, and Zs' and the ogre's are contingent upon another readied action) It really bogs down combat.
Second, it encourages foolhardy play. Would the wizard automatically fireball the room? Would the ogre charge anything that opened the door? It seems that, at a bare minimum there would be some kind of reaction time during which the wizard evaluates where he can place a fireball without catching himself and the ogre makes out a target to charge. Ordinarily that kind of thing is handled by the initiative process but readied actions--particularly ones that are contingent upon other readied actions (which is the best way for PCs to do it since you don't want to fireball AFTER the fighter charges into the room) shortcircuit that whole process by allowing the character who picked the cleverest trigger to go first, regardless of initiative (note that if the wizard readied for "as soon as the door is open far enough for me to fit a fireball through it", he would get to go before any creatures in the room since their readied actions would almost have to be triggered by actually seeing the PCs but the wizard doesn't need to wait to see them.
Finally, it just doesn't make sense. The orcs, the ogre, the fighter, wizard, cleric (he kicked in the door), and archer all knew that there were enemies on the other side of the door. (If they didn't, a surprise round is the appropriate mechanic--not readied actions). They were all waiting for the enemy to become visible. None of them could really (the wizard fireballying through the door as it opens is more than a little silly) act before the door was open. It doesn't make sense that the cleric would always be slower to attack than the orcs and ogre (as he is because his standard action opens the door--he effectively loses his first round action). Nor does it make sense that the combatants would always react in the same order. They're tense, waiting to act. Maybe the fighter is the quickest to perceive the hulking form of the ogre through the dust of the shattered door. (He could roll high on initiative). Maybe the archer's finger slips on his bowstring or he couldn't hold his arrow drawn for quite long enough (he rolls low on initiative). Maybe the wizard fumbles his bat guano (rolled low) or an eager orc began hurling his javalin before the door was fully open (high initiative). None of these possibilities are present in the readied action model.
Finally, if you allow readied actions outside of initiative order, you'll have people walk through dungeons with an attack constantly readied. By the rules, they'd never be surprised or flatfooted again. They wouldn't roll initiative either.
It's much better to do what the system was designed to do and roll initiative (and allow readied actions) only when the PCs and monsters are able to interact with each other.
MerakSpielman said:
On their initiative, the monsters could have readied an action to do something nasty to the PC, yes. In my example, they didn't.
And what if the door isn't there? It's a short section of hallway with a turn at the end and some critters around the turn. The person who announces they charge around the corner might not be the first one there, the way I have it in my first post.