I got my PhD by publication after 30 years of research in industry. Had to go to Holland to find a willing prof. Actually he offered once he discovered that I hadn't got the covetted title. Main motivation was pride: I was fed up with the shame of having to own up that I was not, in fact, a Dr.

I felt I was cheating a bit. I was older than half the jury & have given dozens of talks, so it was no big deal.

Personally, I feel that PhDs are way over-rated. Lots of folk without them are brilliant and lots of folk with them, well, you wonder how they managed to get one. Once you've got one, you are cut out of most jobs, so think hard beforehand.