What the barabarian should have done was throw the half-elf Bard through the door.
As for DMs readying actions before 'combat' starts, well it all depends on what has happened before hand, how much the monsters know, and the situation at hand.
Imagine the PCs have busted down a door (making a hellish racket doing so) and slaughtered a few unwary guards. But they fail to notice the Gnome skulking invisible in the shadows. They also fail to notice he slips out of the room, down a short corridor to the mess hall, where the rest of the gang is having lunch. The PCs took a few hits and wasted some encounter powers in order to take down the guards fast. They have no interest in rushing in any further. So they take 5 minutes to get their breath back. Pretty 'standard scenario'.
So what do the Gnome and the now alert guards do during those 5 minutes... well they all get their weapons from the armory off to the side of the mess hall, darken the torches at the back of the hall, where they turn the tables to give themselves some cover, throw the benches together near the doorway to make a make-shift barricade. The captain whispers for them to ready their bows: ''Anything that comes through the door, shoot it... shoot it dead!''
So that is what they are going to walk into... Action, reaction. The world is alive and dynamic. Your not going to catch the monsters with their pants down every time just because you have high dexterity. And any rules interpretation that dictates actions can't be prepared until combat begins regardless of the circumstances is at best absurd.