Evolution doesn't explain our using 1/10th of our brain

Claim

According to evolution, we don't evolve organs that aren't adaptive. Therefore, evolution doesn't explain our using only 1/10th of our brain.

This claim is itself based on a misleading assumption. There is no part of the brain which is never used. All of the brain is "used" all the time, and the percentage of neurons firing at any given time varies wildly. It's more or less true that no more than 10% of the brain is used at any one time -- the thing is, it isn't always the same 10%.
Those who invented the claim seem to think that it is desirable if all neurons fire at once. But this is more or less what happens in a grand mal epileptic seizure. Desiring a 100% neuron usage is like wanting only ones and no zeros in the memory in your computer - it just doesn't make sense.
If this claim were true it seems more to condemn intelligent design than evolution. Evolutionists readily admit a number of characteristics of living things that, on the surface, seem to make little sense. But, on the other hand, a 10% usage of such an expensive-to-maintain piece of tissue would seem to be rather poor design.
This urban legend may have also been derived from the fact that only one out of ten brain cells are neurons. The other 9 out of ten cells are the various kinds of glial cells, which serve to protect and nourish the neurons. The implication that humans could, but for some inexplicable mechanism, be 10 times smarter than we are, is false.
Those who use this claim fail to realize that, even though only 1 out of 10 brain cells are neurons, neurons actually make up about 50% of the brain mass.

Radford, Benjamin, 2000. (see above)
Wanjek, Christopher, 2003, "10 Percent Miconception, 90 Percent Misdirection: The Brain at Work" a chapter from 'Bad Medicine', John Wiley and Sons.