This information applies to all forms, all martial arts, and all drills. To deal with the initial arguments about not having a stance so that you can respond to any situation, I will say a few simple statements.
1. A stance generates and supports power.
2. All fights end because of the proper application of power.
3. When you are in a stance, every movement generates some type of power, whether you have the skill to take advantage or not.
4. It is easier and faster to redirect power than to generate that power.
None of this takes into account an ambush. Any person who ambushes you will have watched you and tried to take advantage of a perceived weakness at a moment when you are not aware. The first strike is limited only by the truth of their perception and their skill at timing. As such, responding to an ambush has more to do with any flaw in the attackers rather than a skill which can be built. The only benefit, as far as ambushes go, is that any increase in skill increases the possibility of flaws in the attackers decisions of what to attack and when to attack. You can train to ambush, but to counter an ambush you can only continue your training as if you didn't care about ambushes. Some people think you can be trained to respond to an ambush, but all that does is limit you to striking out of fear and panic. Neither of these is a long term solution and what if the attacker chooses a different ambush and you didn't train against that one?
When you realize that a fight has begun, a few things have to happen. You have to make a decision as to whether you are going to put yourself into a position to attack or into a position to defend. If the opponent made a bad decision about an ambush, you might position to attack. If the opponent is skilled at betrayal, then defense will probably be the first decision. Putting yourself into either position will take time which can only be decreased with training. Early in your training would have been where you learned the defensive posture. Since defense is inherently limited, most styles will have taught very few defensive postures.
Many people joke about people going into a stance before a fight because of the time it takes. The punch line of this is that these people aren't going into a stance as much as showing their school's pre-workout ritual. My favorite is "Don't make me take off my shoes."
I realize alot of this has been about going into a stance. The fact is, you are either in a position to generate and receive power or you are not. Those people who feel they should stay in a weak position and somehow generate power because it takes too long to "go into a stance" are simply those who have not been in a real fight. Fighting a weak opponent in an inefectual way counts as not a real fight. Real fights put people in the hospital. When you realize you are in danger, the first decision is to move to where you can make more decisions. You position yourself at the safest location. The ignorant will say that is outside the fight. The smart know that the fight only has an outside if you win or the opponent sucks ass. Once you are in a position to make your decision, you need to be able to move and make that thought real. Your ability to do so requires time and precision. It is forms that make this possible. It is only through forms that you can remove the flaws of weak movement and therefore create speed and effeciency. Understanding the unique purpose of a specific form allows you to train the reflexes. This allows the fight to end faster, since the moment of decision was months ago and that time is not required during the fight. This deals with the "time" of the fight.
The three dimensional space of the fight requires a little more thought and training. If you have read my other pages on stance work and watched the videos, you may remember the information pertaining to ranges of a given stance and how that compares to the ranges of the other basic stances. This is very important when trying to understand why a given form has the stancework for a given series of movements.
The fact is that attackers go for the easy target. It is just a smart decision on their part. Being in a stance creates physical barriers to many targets while presenting others. Knowing which targets are being presented as well as understanding the "perceived" flaws of a posture allows you to predict where an attack is likely to occur. Obviously, knowing where an attack will actually cause damage increases the speed of any defensive decision. You don't have to be ready for anything since there is always some physical limitation in any altercation. You simply have to know where the likely targets are and where an opponent would have to be to take advantage. It is a generalization limited by actual training, but the truth of attacking is that you can punch and kick from any posture. It requires less training to attack since nearly anything can hurt the opponent. Just pick what you are already good at and learn a few targets. A form has more depth than this. A form teaches how to attack while maintaining the capacity to defend and leads the opponent into your attacks. This will be illustrated in video since still pictures don't convey the actual timing and placement of martial movements.