Of course, in the OJ trial, it was Fuhrman pleading the fifth. Nobody thought he’d killed Mrs. Simpson, so in this case the fifth helped the defense.
the problem is, they ask something like “have you ever planted evidence?” This guy had been a gung-ho detective in a big city - to me, that sounds like a fishing expedition… (“Goes to character and pattern of behavior, your honor”?) It would be like asking “have you ever told a racist joke” or those in-depth hiring interviews, “have you ever taken anything from your workplace”, or “have you ever stolen anything”. it’s a no-win situation. You either admit that yes, meaning you helped yourself to office supplies or told that KKK joke, or you lie.
I don’t know about the Casey Anthony thing, but I suspect that dredging up something that made the guy plead the fifth was just a ploy to help discredit a prosecution witness. Whether the guy was taking a nature walk or poaching gators or stashing his drugs would be irrelevant to finding a body, and so really should never have been a matter for testimony. (If they went through that much effort to keep it quiet, one wonders if they were straying into police informant territory, on-going investigation, or something else hush-hush.)
the point of the fifth is that unlike the French system or a Grand Jury, the court can’t force the defendant to take the stand and answer interrogations. (In Grand Jury, they are free to plead the fifth, but I don’t know if that is allowed to influence the decision to indict?) Basically, the prosecution must prove the case without coercing testimony from the defendant. Of course, there have been several threads on this - once a defendant begins to testify about something, they can’t stop. I.e. can’t say “I was watching TV with my mother” then not answer what TV show, tell me what happened, why didn’t you answer the door to pay for the pizza at that time, how did you get there, etc.