That's a good point, and it's often quoted to explain the fact that Ph.D. graduates often make less money (or have higher unemployment rates) than professional

That's a good point, and it's often quoted to explain the fact that Ph.D. graduates often make less money (or have higher unemployment rates) than professional degree holders in the same field. The assumption is that the Ph.D. holder will jump ship at the first offer of a job at his educational level, so he's less reliable as a long term employee.