The arguments about whether there are secret techniques can go on ad nauseum... clearly, there was information witheld from the general populus of a school until they were "trusted" by the teacher because of the methods employed in teaching martial arts in the Orient at the time of their development. Word of mouth and "in person" teaching (usually without the benefit of books, tapes, and scrolls) made "secrets" inevitable. It also had something to do with the student's ability to understand the secrets.
I've taught my senior student much of what I know, and when it was being done, I told him that he wouldn't understand some of the information for at least 10 years if he continued to practice. He's now in a completely different art, and he still comes to me and says "hey, I remember when you taught me such and such, and it just "came to me" when we were practicing how that applied to what I'm doing."
The ability to understand some information is just as important as the transmission of it in some cases, and it's not because the student is a dunce, but because their technique isn't at a level to integrate it into their practice. Since kata was the preferred method of teaching karate, it makes sense that the old masters would integrate movements that could be changed slightly later to "reveal" their secrets to trusted students while everyone else just got the "vanilla" version.
I recently watched a DVD with a current "master" showing bunkai of a particular kata that I was familiar with, and it was clear to me at what point he started "making it up as he went along". His statements started turning to "you can do this, and you can do that" rather than being specific on the nuances and changes that should have been in the kata.
That doesn't make what he was teaching "a secret", but rather a failure to understand the information himself. Over the years, I've seen a lot of this, and it is prolific in the MA community now. Many of the "secrets" aren't secrets at all, just failures of information transmission.
This isn't a direct quote, but close enough to make the point as to why there were secrets...
"The art of war is found in the ability for deception"