One can for instance predict the weather without simulating the path of every molecule and atom in our atmosphere ... The human brain can be simulated in similar detail.
Ah, so it can be simulated to a rough degree of accuracy, but no better? And you can guess what it will do some of the time, and other times it will behave totally freakishly and unpredictably?
Not a human being I'd want to hang around with. Keep it away from sharp implements.
Humans are unpredictable anyway, having approximations will just create different random numbers. A jumble of molecules approximated is another jumble, what makes us intelligent is our brains abilities to turn the random numbers it generates into intelligent thought. For example one way to calculate the square root of a number is to generate a random number between zero and the number to be square rooted, if you square the random number selected and it turns out to be larger than the original number, you select a random number between zero and the previous number that was squared, you keep on doing this until you come to a close approximation of the square root of the number.