Re: Little of both
A national database would provide researchers, government, pharmaceutical companies, and others with tremendous insight into all sorts of things. For example, they could know, in real-time, when and where people are getting contagious diseases like flu, measles, or mumps, then act accordingly. It would also help combat things like Ebola and MERS, as well as cancer. As you say, @pfretty, it would be vital for buy-in that any and all participants reassure the general public about the sanctity of this data, that it's truly de-identified, and secure. Without those valid assurances, then the repercussions could be dangerous (as in some people might avoid healthcare, lie to clinicians, etc.).