Interesting points. I always thought the 5th was more aimed at protecting someone from a crime or evidence of a crime they committed while witnessing another. So if I witness a murder while driving a stolen car, the Question "Where did you get the car?" is not relevant to the murder case, but if you admit it you open yourself up to an investigation tangential to the real line of questioning. Might not be the best example but there are others, like witnessing a murder while buying drugs. What were you doing there, should not be part of the testimony of the murder trial. My two cents.