There may be a very good reason why we don't use all of our potential 'brain power'. I don't think I would rather be an Autistic Savant than bumbling, forgetful old me.
Just being 'normal' must be an incredibly difficult task which actually takes up most of our 'thinking' ability. I don't think this has ever been quantified. As a society, we are very 'fussy' about who we will accept and who we won't. Even being a bit 'preoccupied' with an academic or logistical problem can make us behave in a way which is marginally unacceptable to other humans. How many genii have been looked upon as odd?

I think you're right. I live in the Netherlands (I know my English sucks, please forgive me) and I just visited a museum a few days ago. There was an exposition about the human brain, were I was told that about 80-90 % of the human brain capacity is used for behaving right. In the world we live in that's obviously a very important thing. And because it's not even possible to measure how much 'MB' a human can remember, it's totally impossible to find out how much MB is stored to behave correctly. But it's quite a lot, I suppose.