Dale W. Callahan wrote an intereting article in IEEE Potentials 2001, entitled "The Ph.D. process."
"The bachelor’s degree proves that you can be trained; the master’s degree indicates some expertise in your field, and the Ph.D. indicates your ability to become an expert in any field."
Rob N. Candler refered to his professor's words in "Stuff most students never ask about grad school", IEEE Potentials 2005.
"You’re given a toolbox at the beginning of your undergraduate education. Each class you take gives you a tool for your toolbox. So, the question you’re really asking yourself at graduation is “What do I do with all these tools?!?” I think an M.S. gives you a few more tools, while allowing you to use some of the tools you already have. A Ph.D. enables you to build new tools."