I think the second pilot is there for much more than computers can replace, at the moment anyway. What about bathroom breaks, sickness, or even incapacitation? Bouncing your ideas off the other guy, backing each other up, etc.

Once a month the guy I'm with keeps me from doing something very stupid that I didn't notice on my own. Computers aren't that smart yet, and since my current airplane is based on computers as old as I am, the airplane I'm flying in 20 years might be based on today's stuff.