I have discussed the same topic with colleagues over both PhD(c) and DNP(c) use. My opinion of not using this has been shared by the majority. A former professor of mine had a PhD in physiology who’s wife never successfully defended her dissertation. She continued to use the PhD(c) designation before and after dropping out post 2x dissertation failure. It was obvious he was resentful of this. Clearly there were problems with someone failing defense twice, but I digress.
My own opinion is use only official titles earned. Unless like the poster above, PhD(c)/DNP(c) are not awarded credentials at any institution I’ve studied at, therefore one should not use them. Personally, I wouldn’t use it anyways, as they come off as pretentious. Coursework is coursework, defense is what culminates the degree in it’s totality. Using the PhDc/DNPc, and ABD for that matter is nothing more than an ostentatious display.
But again, this is my own opinion.