This is a subject open to debate and that debate will always continue regardless of the answers either I provide or others provide. It is just the way this subject is and that will always differ depending on the perceptions of each individual practitioner.
Drills are a great teaching tool. The military have used drills and drilling since time began. It is one tried and true way of training the military who have yet to achieve experience in battle. What changes is the individual combatant when they start to accumulate experience then those drills become lost in the reality of battle, fighting and today defending.
In today's martial arts rarely do practitioners get to test their drilling practices in live fire, out on the streets in a violent situation or even in the sport side rife with rules and requirements under the auspice of safety, etc.
How do you know your drilling and practices will work if they are not tested in the heat of battle? You don't. Drills were created by those experienced combatants long ago so that novices, inexperienced, can learn some foundation or basics to work with when they do enter into harms way. That transitional phase is crucial to combatants, especially military, because that is the point where the greatest loses are experienced.
I remember a Viet Nam veteran Marine I worked with when speaking of newbies who arrived in country and how they would be treated a certain way until they accumulated enough experience that they might survive without causing their fellow Marines to possible die along side them. Ignoring the good or bad to this one can see that the initial period of entering combat is critical to both the newbie and the veterans - for survival.
Is it conducive to lasting past that initial period to rely solely on those drills, i.e. the drills and kata of martial arts, steeped in patterns, combinations and rhythms to teach a person how to survive violence without providing some means to get closer to reality? In modern times and I suspect even in the past those who trained in such a manner instinctively knew that they needed to do something that would enhance the teachings of drilling and kata, etc.
In ancient times it was not hard to gain experience in combat for the times provided many opportunities to test your skills. Japanese tales abound with how samurai would challenge one school or another to test their skills not to forget that during those feudal years they got to test their skills in combat. Today, we seldom, if ever, get to test our skills in true violent defense.
Is is possible relying solely on drills and kata to achieve fighting ability is less that optimal? Is it possible that one does not voluntarily go into a scenario like fighting, etc. as a part of human survival instincts thereby making the idea of drill effectiveness more acceptable? What happens when a violent adversary does not adhere to the patterns and rhythms of your drill? What do you think your mind does when the adversary is not following the drill or kata? Is going into a training regimen beyond drills and kata resisted because it would mean leaving the comfort zone of your perceived safety and security?
I have learned over the years that many traditional marital arts believe and rely heavily on drills and kata for their combative or defensive skills. I have learned over the years that many martial arts consider competition and kumite sufficient for their combative or defensive skills. How will any of them discover the validity of what they learn and practice if they don't learn through experience in violent encounters?
Even if martial artists are exposed to reality based training regimens how will they encode it into instinct if they don't do some sort of repetitive training using what they learned and still not know for sure if it will work for them until they experience violent encounters where they can utilize the lessons learned. This how will always be a bane of contention and my theory is that the only way to truly find out if what you train will work is to test that training by the fire of combat - just like the military in combat.
In closing, the answer to this question resides solely in the hearts and mind of each individual martial artist. No matter what I say, what others say and what is professed as the ultimate defensive system it will only have meaning when that individual finds the truth through combat experience.