These programs are analogous to the MD/PhD programs that train Physician-Scientists. PT/PhD programs range from the interdisciplinary BIOMS program at UDel to PhDs in Physiology, Engineering, Movement Science, and yes Physical Therapy.
The idea is to give an individual both doctoral level clinical and research training in hopes that they further the scientific base of the profession. We all can see the significance of this in evidence and outcome based practice. In addition, research trends are now moving to multidisciplinary projects, and in medical research, Physical Therapist trained researchers will obviously hold a special niche in rehabilitation.
Does it increase pay? Well, you get paid according to the field you work in. You are paid as a scientist, professor, clinician, or whatever private industry pays if you end up there.
BUt, if your goal is to be a clinician, the PhD is worthless for you. Do a residency program, or two...