I'm PhD student beginning my third year. I plan to graduate at the end of my fourth year, leaving me with about another two years. However, at the end of a productive summer of research, my PhD supervisor has mentioned that I'd be on track to finish considerably earlier if I want to. My program uses the "three papers" dissertation model, so at any given time, it's pretty easy to gauge how close we are.
In fact, I do not want to finish early because I am really enjoying doing research with my supervisor and feel that I'm receiving outstanding mentorship from him. Additionally, four years is already fairly short for my program (which is usually done in five years), so it doesn't seem that there is much additional benefit to finishing before year 4.
Instead, I'm considering asking my supervisor if I should defend early and then become his postdoc for whatever remains of my planned four years. I imagine the benefits would be:
Since I plan to apply for faculty positions after this, I would have nominally "done a postdoc" instead of being fresh out my PhD.
I'd be off the hook in terms of departmental responsibilities, like TAing, for my fourth year.
Are there other advantages or disadvantages to defending early to become a postdoc? Also, are there advantages or disadvantages for my PhD supervisor?
(I have reason to believe that postdoc funding would not be an issue, so perhaps let's not focus much on that.)