Recently, a previous student of mine consulted me the possibility of applying to other PhD programs while deferring a PhD offer. He was an undergrad at my institution; he got a PhD offer last year but decided to work an extra year before starting his PhD. We had a good relationship while he was working in my lab, so I hope to help him out by seeking some information on this.
SE (i.e., this platform) seems to have very mixed opinions on this. From the answers I've read, some say it's fine while others bring up ethical reasons against this behavior. However, almost all answers (some even from the same people who are against reapplying during deferral) support a current PhD student applying for another PhD program (at least nobody said this is unethical). This, to me, is very puzzling because in the 2nd case the student has already costed not only a seat but also actual funding from the program.
If reapplying is allowed for current PhD students, why are people against it for current deferees? To the best of my knowledge (including in my own program), students who choose to defer are rarely required to sign a binding contract. From my perspective, they are basically PhD students who are on leave.