Bob,
First, I have a caveat. I know something about a number of Okinawan kata (and their Japanese derivatives). I do have some familiarity with many other fighting systems. Unfortunately for those with forms and kata, I have chosen not to try to better understand these movements. As I said above, I have abandoned dozens of kata in my own system. (Although I do have some understanding of the movements for many of them.)
I have seen Kempo kata, but I confess ignorance regarding the inner workings of them. So although I have been using the term kata, I have been using it in a specific context, kata that have evolved from those practiced in Okinawa 90 years ago when Itosu, Kyan and Higaonna were teaching. Mea culpa if this doesn't fully apply to Kempo.
The reason that I make this qualification, is that I have firsthand knowledge of the effectiveness of Okinawan kata, and why just a few can serve as a complete foundation for a fighting system.
The issue comes down to the breadth of application contained in the movements of Okinawan kata. I just don't know whether there are comparable kinds of applications within other systems with which I am not completely familiar.
To me, perhaps the most fascinating aspect of Okinawan karate is that some movements are really like swiss army knives. They can be used against many different kinds of attacks. Sure, there are small differences between the actual movements in these applications, but the general pattern can be repeated.
In the applications I do, I stick closely to the kata, and not only are there blocks, counterkicks and strikes, and all sorts of locks, but there are takedowns for every application I practice. Against a big attacker, my goal is to put the big guy on the ground so his relative advantage in mass is greatly diminished. And I can just follow through the steps of kata, and do that hundreds of times, with the handful of kata I teach and practice.
In the applications I do, the "turns" (and pivots) are an integral part of the technique. Everything you do benefits from good body mechanics, bringing your body into the technique. You can do that by moving forward, or by turning, and many forward movements have pivots as well. So you turn to block and counter, then continue with your combination by perhaps turning to lock, (and perhaps counter again) and continue again by perhaps turning to throw. I say perhaps, because sometimes the forward movements are used for locks and takedowns. In Okinawan kata, there are turns and forward movements, and both come together.
So lets say a kata has starts with turn A and proceeds, each turn and it's associated forward movemens being a letter. AB or ABC would be a technique, (and perhaps, though rarely, ABCD) BC and perhaps BCD are also techniques. So is CD or CDE, and DE, EF and so on. In some cases, movements modeled on the body mechanics of one "combination" can be used against a variety of attacks, and in rare cases, perhaps 5, 10, 15 different kinds of attacks. (For example AB, or ABC at the beginning of Pinan Shodan and Pinan Yondan.)
Please note that this is a very unorthodox approach to kata interpretation found in Okinawan and especially Japanese karate today. Many, many systems use turns primarily to face an opponent coming from a different direction, and then complete the technique with a single counterstrike/kick found in that one "direction." However it is what, in my mind, makes kata practice so useful. It is not a bunch of techniques strung together, but a bunch of overlapping techniques strung together, and why the practice of the kata is different from the practice of individual techniques or components of the kata.
Again, I am referring to Okinawan kata, but this is why just a few kata are so hard to master. Because if you really want to use the kata to its fullest, you have so much work ahead. Let's say a particular combination can be used in 10 different ways (not the norm). Then I do believe why most MAists could see why it might be a good idea to adopt those movements fully into one's defensive repertoire. And for that you need thousands of repetitions of kata, lots of bag work and tons of partner work.
Now let's say you decide to go forward with that plan. How long would it take to achieve in the typical karate dojo? I would venture to say, from experience, several years, just for one kata. And if you wanted to, for a variety of reasons, introduce kata say once per year, it would take far longer.
Even more important, if your school were not set up for this kind of repetition, but you needed to do this on your own, outside your dojo, it would take many years.
That's why I advocate finding a few kata that work for you, and focusing on just them. But again, when it comes to Kempo please recognized that I don't understand the kata. Maybe more kata are needed to get to the right number of combinations good for self defense. Some Kempo kata I have seen are relatively short compared to many of the Okinawa forms. (There are relatively short Okinawan forms, Pinans and Naihanchi being examples, but most kata have 15 or more directions.)
But then that is a whole other story. How many combinations do you really want to practice for true self defense?
