Out of curiosity, why do you consider expanding career opportunities to be a negative? Isn't that a big part of ed at all levels?
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.
I haven't tried any other job outside of academia since I started in the field, so it's a bit speculative, but I think that were I to enter the business field, I'd be better suited have gone through graduate training. I think a lot of people who already don't have good social skills go into lab work, and those already lacking social skills certainly don't improve them by spending five years with their head in a book or a microscope. But for the well adjusted, I actually think going through the process of self-doubt and discipline necessitated by finishing a dissertation can be character building, independent of the actual skills learned (which certainly won't be directly transferable to most fields).
I was traveling for work for a couple of weeks, so I've only just seen your reply now. I agree completely with your comments -- graduate training can increase how good you are at other jobs and be character building. I don't think it's a particularly good sole/major reason to go to grad school -- one would probably be better served by working in a particular industry instead, where one can character build and obtain relevant skills.