I've been railing about this for some time. I think when people say "we only use 10% of our brains" they're implying there's 90% that never gets used for anything, and it's some kind of mysterious untapped potential.
During a normal 24 hour cycle I believe most of the brain is used in a variety of tasks and experiences as in your list (eat, talk, move, etc).
It makes sense that larger brains are advantageous for survival, but why would we evolve with an enormous extra capacity when it takes so much fuel to support? We may not know the full capacity of the brain, but it makes evolutionary sense that it would not be much more than what we need to survive.
Did you see http://www.snopes.com/science/stats/10percnt.htm?

EXCERPT: "although certain minor functions may use only a small part of the brain at one time, any sufficiently complex set of activities or thought patterns will indeed use many parts of the brain. Just as people don't use all of their muscle groups at one time, they also don't use all of their brain at once. For any given activity, such as eating, watching television, making love, or reading, you may use a few specific parts of your brain. Over the course of a whole day, however, just about all of the brain is used at one time or another."