I agree that in general, expanding career opportunities is a big part of education at all levels. However, I don't think that this is a particularly good sole reason for pursuing a Ph.D. -- my general impression from colleagues is that having a Ph.D. can be a hinderance on the (outside of academia) job market. I'd think in general (and of course, your mileage may vary...) that if improving career prospects is your main aim of graduate school, you're better off leaving with a M.S. and getting relevant job experience, instead of spending the additional 3-5 years on a Ph.D. (which is what I usually think of as "graduate school").
Another point to bear in mind, I think, is that if you're interested in academic jobs, it's also really worth being realistic at all stages of your chances. Very few Ph.D. students make it through to full-time academic positions and there is really no shortage of extremely well qualified candidates. I'm not advocating against this path, just it's worth having some back-up plan and gathering "transferable skills" for outside careers whilst in the academic pipeline.