The answer, of course, is yes, they should have quit. But the next question is, having quit, would you vote for any of them again?
But that confuses the issue. For Nixon, Weiner, and Wu, the reason they quit is identical to the reason you wouldn't vote for them again. They were forced out of office due to an intolerable character flaw, and that same flaw would prevent their reelection.
But for Palin, the reason she resigned is identical to the reason you would vote for her again. She had sworn an oath to the people of Alaska, to fulfill her duties to the best of her ability. The legal cage devised against her prevented her from fulfilling those duties as promised. The only way she could keep her promise was to escape that cage, and the only exit from that cage was to leave the office of Governor.
So yes, the reason she quit points to an admirable character and I would not hesitate to cast my vote for her or any politician like her, who takes their promises so seriously they would rather risk ruining their political career than break that promise. We need more Sarah Palins. Many more.
Answer: some yes, some no. It depends on the person, and the reasons for the "quitting". Each individual - IMHO - should be treated based on the merits (or lack thereof) of the situation....not be painted with the broad brush of derision in the attemt to prove and unprovable point.
If I may offer a too long bit of bumper sticker philosophy, but one in which I place great credence...
"Perseverance is the most overrated of traits, if it is unaccompanied by talent; beating your head against a wall is more likely to produce a concussion in the head than a hole in the wall."