In pretty much all subjects, PhD programs produce far more PhDs than will be needed to replace the faculty at research universities as such faculty retire. Of course, some of the “excess” PhDs go to LACs (not many, because they are small), non-research universities, community colleges, or even high schools for faculty positions, and some of the others become adjuncts. But that still can leave “excess” PhDs, even in the sciences, who must look for non-academic jobs. See http://www.nature.com/news/2011/110420/full/472276a.html .
So attending an unfunded PhD program in anything is a high risk endeavor in terms of potentially a lot of debt that one would have difficulty paying off in the weak market for PhD graduates.