The need for a second pilot in the flight deck cannot be simplistically measured only for the task of monitoring the PF. He is such a valuable asset when things get messy in the cockpit (and things do get messy sometimes), especially when flying to certain "less standardized" countries and decisions have to be taken at the end of a long-haul night and weather is minimal (as well as information) and flight controllers are unable to express themselves in fairly understandable English...South American destinations, as well as some parts of African, Far and Middle Eastern destinations, come to mind. Now, add to the equation a contingency fuel of only 3% (the minimum legal) and you'll get a pretty good picture of what comes next!
One can substitute a pilot by a set of computers, but in the end, computers are unable to gain experience, and especially are unable to improvise, in order to save your day.