(Playing Devil's advocate). I agree to an extent. The rules are to be used as a guid not the end all be all. Plus I disagree with modifying the rules to save a DM's butt. I will give you an example and Moose will attest to this.
When it comes to rolling 20's not even on my best day can I beat Moose. He makes passionate love to his dice, so when it comes to called shots I have to take liberties. I had a new module we were doing. I spent!the money for it, spent the time reading it and preparing for it. The first encounter with the boss during the first session he calls a shot to the head. Bang the guy is dead not even an hour and a half into the first session. Now what! He says that I am limiting him or cheating. Sometimes I am its true. I have to do it so I can progress the story. It's not easy setting up a game only to have it finished an hour into the session.
The rules are guildes to follow, to structure the gaming world. It's up to the DM to apply, and allow some flexibility how he sees fit. Whatever helps the progression of the story. Yeah I am a dick for no reason that you are aware of but for me its a lot of work ruined because you called a shot.
Here is another example. In a recent game I was running 2 of the party members fell into a trap which dropped them into a pool of oil. The boss pulls out a torch, places the flaming end over the rail of the balcony that they were on and was using one finger to keep it from dropping into the oil below. The guys climed out of the pool went back in, in an attempt to clime out the shaft they fell through, decided that they couldn't or they would be killed, but only one of them actually decided to get out of the pool on a platform. The fight was supposed to be on the platform so they could get some gear and weapons, but one of them was too busy on the phone or with his other gaming cards that when it was his turn he forced a fight on the balcony forcing me to drop the torch and nearly killing to characters who were paying attention. I gave them the first shot because not only did they have initiative but I wanted to teach them a lesson. I should have slain the two guys in the oil pool and killed the guys off in the encounter but I let them get away using a spell I knew that he wasn't supposed to be able to use for another day. I did it not only to teach them a lesson but because I wanted to progress the story.
Now to keep the story going I have to make big changes on the fly. But if I were to follow the rules I would have been forced to kill the whole group. I have to have a lot of flexibility or the chaotic nature of the players ends the game ealry. I also now have to exact a consequence for allowing them to escape in a rule breaking way.
See flexibility