It is true in the sense that we only use about 10%, at any given time, for some definitions of percent. People just fail to understand that the other 90% is just not useful, rather than unused potential.
Is it even usable at same time? There is some level of plasticity, but in general certain parts activate for certain tasks and you can't really make it use more parts at one time.
Not saying that only X% of our brain is being used, but what it essentially says is that our brain is power-limited, and therefore, it has to be selective in which parts to "turn on". It makes a lot of sense, there are few things in life more important than energy and power management, and human brains already need lots of both.
