Do we actually need more PhDs?
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.