She passed the time quietly in a world of her own in which she was surrounded as far as the eye could see with old cabin trunks full of past memories in which she rummaged with great curiosity, and sometimes bewilderment. Or, at least, about a tenth of the cabin trunks were full of vivid, and often painful or uncomfortable memories of her past life; the other nine-tenths were full of penguins, which surprised her.-Douglas Adams, The Long Dark Teatime of the Soul
So in case anyone didn't already know, this is a myth. It's not true that we only use 10% of our brains, nor is it true that only 10% is active at any one point, nor is it true that we understand, or are conscious of only 10%. When I googled the words "10 percent" and "brain" I didn't find a single credulous article until the third page. So I think saying more on this myth is just overkill. Instead, I'll go off on a tangent about how skeptics deal with truth.
Anyways, I think the reason the 10% myth persisted for so long is that people like the point it is meant to make. "Everyone has a lot of hidden potential," is the message. Who are we skeptics to take away from that? It's certainly difficult to think of a case where the 10% myth would actually harm anyone. Well, the myth is occasionally used to justify drug use or belief in "psychics" (aka con artists), but for the moment, let's ignore those possibilities. If it's not hurting anyone, and if people actually find meaning in this myth, why burst their bubbles? Isn't this a slippery slope in which skeptics will start to reject all of life's little lessons and become bitter-hearted cynics?
Not really, no. But I do think we should reject life's little lessons when it turns out they're wrong. If the only way to justify a life lesson is through lies, well then maybe that lesson is wrong. If it requires pseudoscience to inspire people to do something, then maybe people shouldn't be inspired. If people need a myth to find meaning, that meaning might be false. False meaning can still motivate people, but who in the world wants to be motivated by lies?
So is it true that all people have hidden potential? I think so. But it's not because we only use 10% of our brains. Surely, we can think of a good reason without resorting to myths. It is because when we see great men and women, we realize they are not so different from us. That wasn't so hard, was it?