> neurons are enormous and filled with tons of useless junk that isn't used for computation

I just want to point out that some recent research indicates that this might not actually be true. Single neurons may be more complex than we thought since researchers managed to extract significant and complex functionality from single neurons (!). Not to mention glia seem to be important for cognition, and not just glue as we thought... So perhaps not as wasteful and kludgy as we might think :)