A Ph.D. unlike a master’s degree, can have applications beyond moving into a specific job or career that requires a Ph.D. Most people pursuing a doctoral degree are looking toward a specific degree for a specific application but a Ph.D. has an aura about it that makes it valuable for purposes beyond its original intention.
If you want to publish articles or books, your credibility soars with a Ph.D. even if your degree is irrelevant to the subject matter of the piece. If you want to get a book published, get a Ph.D. Get it anywhere. Get it in any subject. As long as you can stick those letters after your name, you are an instant expert, almost a genius. It will also gain you entry into many other fields of endeavor that also may have nothing to do with the subject matter you studied.
On the other hand, when applying for a specific position, your Ph.D. may not be worth squat. I found myself in the position of interviewing people for a job as a conselor at a residential facility for mentally ill adults. I wasn’t the director of the facility but was asked to interview prospective counselors. I had a master’s degree myself but found myself interviewing some individuals with a Ph.D. in psychology. We didn’t hire any of the Ph.D.s because none of them were as qualified as others who had lesser degrees but more experience.
So if a Ph.D. is compatible with your goals or if you think you can get more chicks or guys with a Ph.D., go for it. But be sure it is going to do for you what you want. If the jobs aren’t there, you could be just another Ph.D. intelligently and expertly flipping burgers.