No system is going to be perfect, and turning it completely over to a computer would result in more balls and walks because (imo) the majority of bad calls are balls that are called strikes rather than strikes that are called balls. And the last thing baseball needs is more walks (longer games, etc.)

So I say give the ump the benefit of the doubt up to a point. Maybe 2 inches or something. Any call the ump gets wrong by 2+ inches is overruled by a computer. Otherwise, the ump's call stands.