For us beginners, there are only two Bunkai:
Kanshiwa and Seisan. I can't really comment on any others.
I always liked Kanshiwa Kata because it HAD a Bunkai, but in fact as time went on, I wound up learning Bunkai (applications) for every Kata along the way.
Of course, these are not formalized in any way, so this is only in the context of my particular training.
We learn the formal Kanshiwa and Seisan Bunkai because, I assume, we have to eventually demonstrate at least one commonly-agreed-on version of the Kata techniques in order to "pass a test".
This is the requisite material we learn for presentation/demonstration only, in a formal testing setting. Something we all can have in common.
I call this material the "Uechi Compulsories".
But outside of that there is, as RW is saying, a rich world of applications which are the real meat of the Kata.
Here's my take on this.
Kata is meant to ingrain definite habits of movement, but not to ingrain any specific applications of those movements.
This would be the warning of the Okinawans RW alluded to.
Karate training is not about compiling a look-up table of specific responses to specific attacks.
It's about forming general reflexes and attitudes and developing the physical wherewithall (conditioning) to make them (hopefully) work.
However, you have to demonstrate SOMETHING to show students where this is going, and so the formal Bunkai provides some common ground to "bust a move" we all know.
For example, I could probably land in Edmonton, go to RW's school and say "hey, let's do a Kanshiwa Bunkai!" and everyone would probably know what the heck I was talking about and we could do it and compare notes.
Sure it'll be different, I'd expect that.
But we'd have a common starting point, so I wouldn't have to train for 6 months there before I could even participate in a basic class drill.
To me that's the purpose of the "compulsories"; they are common ground and a yardstick that a Sensei can use to sort out who can do what, at what level.
Now, if RW were to say to me, "OK, show me some other things which that concept could be used for." Well, THEN I could do that in an illustrative way, but not setting up an unsuspecting student in a "standard Kanshiwa Bunkai" and then doing something COMPLETELY outside the drill to make the student look ridiculous, and make the drill look useless.
Seisan Bunkai has an even different tone about it, because it's the Bunkai that Brown Belts learn for the BIG TEST.
So right off the bat, Seisan Bunkai has a tone of "preparation" about it, rather than a tone of "learning the Kata techniques".
There is is increasing pressure to "learn it right", because this will be the "TEST Bunkai"... and so this may NARROW DOWN the focus of the higher level Kata at the very time when the student should be learning to OPEN UP their mind as to the various applications of these "higher level" techniques.
How ironic is THAT?!
I have my own personal way of "compartmentalizing" these conflicting requirements but that's another story.
The music spoke to me. I felt compelled to answer.