Do we actually need more PhDs? A few months ago I attended a panel discussion on non-academic future careers for physics PhDs. The panel was made up of two senior physicists, a science communicator and freelance journalist, and a CEO of a finance corporation with a degree in physics. At some point I raised the question of whether the PhD program is functioning properly, and maybe we should do something about the PhD program before lecturing senior graduate students in theoretical physics on the importance of acquiring skills suitable for non-academic careers to get a job after grad school. To be more concrete, I referred to a Nature article that I had come across a few years earlier. The two physicist panelists blatantly dodged my question. The CEO, who I suppose had received his degree in early 2000s, added a comment along the lines that it is better to have more highly educated people in the society rather than fewer.

Are advisers hiring more graduate students to get more research done and compete better in getting grants, or there is an actual need for more PhDs, at least in natural sciences?
We can make the same conversation for BSc or MSc degrees. There are probably many unemployed people with BSc degrees, as well. We cannot suggest that people should not go for higher education, because there are actually fewer jobs than university graduates.

Your question seems to neglect an important, yet simple truth. That sometimes, certain people want to go for a higher degree, regardless of their job outcome. Education (PHD included) is more than filling resumes or getting a better job but a fulfillment in itself. In this sense, it is the same as learning to play the guitar. Does the world need a new guitar player? No it does not. But you do not learn the guitar to benefit society. You learn the guitar to benefit your soul. 

If the few lucky PHD holders manage to get the job they wanted that is fine. If they did not, at least they tried. Life is not linear and you cannot predict by your actions on what job you will land, even if you do everything perfectly. But at least, you must be provided with a chance to try. In that sense, it is better than there are actually more PHDs available than the available jobs for PHD holders. More people have access to an even higher education and that is (as aeismail said) a benefit in itself. 

So to directly answer your question: Do we need more PHDs? No, we don't. Would the world be a better place if you or I get a PHD? No, it won't. Would we get the job we want with a PHD? Probably not. But that does not mean that I must not have the right to do a PHD and try my chances.