I think you have to try to avoid the situations you described, but I think 99% of the time, bad mouthing someone who is toxic is not going to have these far-reaching negative effects. I have done it in many exit interviews and its never bitten me. I obviously do not ask for references from terrible managers who I bad-mouth in exit interviews, rather I ask for references from strong leaders who I can honestly speak glowingly about and who I develop close relationships with. I think you shouldn't be afraid to speak the truth in most cases and if you've developed a reputation as a good employee who others like to work with, many times the information you provide will be taken seriously. It probably won't be acted upon unless others report the same thing, but I still find it worthwhile. It at least gives them the opportunity to do the right thing.