Even assuming we could do a good brain prostethic, with our current computer hardware and software, what would it be able to do?
What springs first to mind is remembering lots of data, or doing arithmetic fast. But would such a brain prostethic offer any real-world benefits over a human using a laptop or calculator?
OTOH, I suppose you could record signals from someone skilled in something, and then use those to augment training of someone else. Like in The Matrix. But by then we would be bumping into what today are philosophical questions, like "how compatible are different people's mental models of self and of the world"?
Also, what would this mean for e.g. eyewitnesses used in court? In a dystopian world, you could always imagine some criminal gang "programming" otherwise reliable eyewitnesses to give them alibis. Or even a shady government implanting memories of a crime into someone's head, in order to make them confess to that crime.
Or, suppose that a government could literally read your mind and tell if you did something or not. Would you be protected from that under the Fifth Amendment?