I'm inclined to see those sorts of resignations as less decent than the alternative.
I first ran into the phenomenon when my parents put me in a private school(NE United States). The pattern, at that school and the others in the area that I had any familiarity with, was that 'expulsion' was something that happened only to the people who screwed up most blatantly, or who the school really wanted to get rid of, while more desirable or less noxious students would be quietly advised, off the record, that it would be better for all involved if they were to(depending on the point in the school year) either 'withdraw' or not seek readmittance for the following year. School gets them out, they get no disciplinary record and are free to move on, largely unencumbered by whatever it is that there were up to.
In institutional contexts, you see similar things. Peons get fired, real people get to resign when the situation looks untenable.
In something as blatant as Nixon's case, obviously, resignation wasn't going to 'hush up' what would have led to impeachment, it was just too big; but the use of 'voluntary' resignations as a "Gentleman's expulsion" oozes noxious privilege in my mind.