Other sources for the ubiquity of the 10-percent myth probably come from popular authors' misconstrual of scientific papers by early brain researchers. For example, in calling (for technical reasons) a huge percentage of the cerebral hemispheres the "silent cortex," early investigators may have left the mistaken impression that what is now referred to as the "association cortex" had no function. That was far from the researchers' intention, but that is what seems to have filtered through to the public. Likewise, early researchers' appropriately modest admissions that they didn't know what 90 percent of the brain was doing probably fostered the widespread misconception that the leftovers did nothing.
In my quest for the seminal utterance of the 10-percent myth, I frequently came across the claim that Albert Einstein had once explained his own brilliance by reference to the myth--Einstein's enormous prestige, of course, making it unassailable thenceforth. A careful search by the helpful people at the Albert Einstein archives, however, was unable to provide me with any record of such a statement on his part. So it remains probably just another of those instances where promoters with a point or a buck to make have misappropriated the clout of Einstein's name to further their own endeavors.
The 10-percent myth has undoubtedly motivated many people to strive for greater creativity and productivity in their lives--hardly a bad thing. The comfort, encouragement and hope that it has engendered helps explain its longevity. But, like so many uplifting myths that are too good to be true, the truth of the matter seems to be its least important aspect.