In order to express the way I feel about kata, I would first like to explain how I think kata came about and what it was originally used for:

One of the hardest things to do, even for people who have been doing martial arts for a while, is to remember a newly developed technique. 

On many occasions, a fit of inspiration along with a bit of trial & error with a training partner has led me to develop (stumble upon) and make up techniques that I considered to be very good, original and functional. After a number of repetitions, I mistakenly believed that they were so good that I would be able to remember them without writing them down, only to wake up the next morning to have the completely disappear from my memory. It is one of the most frustrating feelings I know!!

One solution is to write them down as you think of them.

I believe that katas were developed with the sole purpose of remembering newly developed and favorite techniques: a number of techniques would be put together in a form (kata) so that the practitioner could repetitively train them without a training partner in order to "brain-wash" the coordination needed to successfully perform the technique (rather like a golfer performing a load of practice swings in his/her back garden before going out on the course). Katas could also be used as a prompt to decide what techniques to practice when he/she did have access to a training partner.

One of the most common questions that I get asked of students is why the katas do not very often  closely resemble the bunkai. I have heard many weird and wonderful explanations to this question but I think that the answer is fairly simple - when the katas have been passed on from student to teacher, some techniques may not have worked for an individual and were consequently ignored or changed.

Also, as karate has been more and more standardised (a very bad thing) kata training has been more and more separated from bunkai training and a lot of the applications have quite simply been forgotten. Also, moves have been continually refined to be more aesthetically pleasing for competitions. There are actually a lot of clubs and styles that don't perform any bunkai training at all.

I don't think that any of this matters very much. I think that kata training is a very valuable training tool.

For a beginner, katas should be used to perfect basic techniques, and improve coordination, movement, breathing, balance, focus, concentration and strength.

For the more advance student, the katas should be used as a prompt to decide which bunkai should be practiced, refined or replaced.

For the very advanced practitioner, the katas should then be re-learned to exactly mirror the bunkai that the practitioner has chosen and perfected. These will be the practitioners own private kata sets and will then function the way katas were first intended to function: a way of remembering and training an individual's own set of self-defense techniques.