The usual answer that comes out of the assembled geek experts is: "only if you have great passion around [subject] and will never be happy doing anything else." This is indeed a part of the answer, and is often motivated by a fear of being stifled by a boring, mundane job. However, I often find an essential second part missing: "do you have a reasonable hope of success in [subject]?"
Graduate school may be about the "disinterested pursuit of learning" for some privileged people. But for most of us, graduate school in the humanities is about the implicit promise of the life of a middle-class professional, about being respected, about not hating your job and wasting your life. That dream is long gone in academe for almost everyone entering it now.
There are a number of well-known critiques of the academic world and its hiring practices, and these are frequently pooh-poohed by the successful academics one finds in the Physics Forums. One never sees career advice from the failures who couldn't successfully navigate the system, although that might be more useful.
But let us return to the purported benefits of great passion around [subject]. One of the benefits of the academic life is the ability to spend your life delving into your chosen subject, teaching, researching, writing, etc. Yet, this genteel image is in conflict with what the academic world does, and is expected to do. Lurking in the background of all higher eduation in the US is a vocational model linked to national economic output. We need more PhDs to remain economically competitive. Research universities drive change and innovation. More education means higher earnings.
Few truly want to support yet another doctoral dissertation on Proust, and the business of education is expected to provide a cure for cancer soon. But at the same time, no one seems all that keen on focusing young people into hard science and engineering for the economic benefits, despite our expectation that universities will magically provide us with the technology we want. The romantic image of the life of the mind allows the system to take advantage of idealistic young scholars.