Actually, I do think that both Cain and Lott should have stayed. Accusations against Cain were never substantiated and were in fact being disproved, and his dropping out was voluntary; Lott's statements seemed dumb (though were taken out of context), but in any case the call was for him to step down as Speaker, not resign his seat (a fine line perhaps).
I'll even say that Harry Reid's moronic statement about Obama not having a black accent, as off-color as that was, should not have compelled him to step down or resign.
I will concede, though, that public figures should rightly or wrongly be careful about what they say. But if a public figure legitimately (!) apologizes for his statements, then that should certainly be taken into account.