I learned later that the Columbia DDS/MPH awards a full or partial scholarship to a dental student. This may or may not be true. But honestly since many programs (Harvard, Columbia, Berkeley, Hopkins, etc) offer 11-month MPH degrees to individuals with doctorates, there isn't much incentive to pursue the dual track. If you want to specialize in public health dentistry, then you must have an MPH first (only speciality of dentistry that requires a masters degree in addition to a DMD/DDS).
An MPH is a very rewarding degree for someone wanting to work in academic dentistry, research, or simply community based dentistry that works on a population level (e.g. social safety net programs, rural clinic dentistry, epidemiology, etc.).