This is a comment that I've heard more often in the past month, for some reason or another and so would like to redirect readers to a much more eloquently written blog post about this very subject:
However, if you would like the summary, here it is:
Brain injuries, however small, can cause gross loss of function. Even if they are dispersed, such as Alzhemier's, a significant decrease in neuron number and connectivity causes increasingly severe insult to our cognitive and autonomic brain functions, depending on the focus and rate of damage, of course. If we really only used 10%, we would have 90% of real estate that could be damaged, or could take over some functions. This has not been seen in any medical case. Ever.
We know to good detail the brain's "map", it's metabolic cost to the body (20% of glucose at rest!) and it's evolutionary antecedents. All of these swiftly debunk this 10% brain myth. It's astounding how many people regurgitate this myth when a simple cross-examination of even daily life disproves it. I shrug, it's not something that not knowing or knowing puts your life in danger, and its probably not going away either. But at least I feel like I've done my part.