That's an interesting throwback to the days when all astronauts were test pilots, but it's kind of stupid nowadays, for reasons Robert Zubrin points out in his Mars Direct plan. It's simply easier to train a qualified engineer or scientist to be a pilot than it is to train a qualified pilot to be an engineer or scientist--and on any mission that's at all interesting, it's silly to waste space on an astronaut who isn't an engineer or a scientist.