In particular, if a person's behavior or circumstances conflict with the advice, it becomes important to evaluate why that conflict exists. Is the advice fundamentally sound and there's something wrong with the advice-giver, or is the problem with the advice itself? There may be some subtle implementation detail that you overlook in your theoretical "ignore the person" evaluation, which is actually a fatal flaw with the advice (for example, it may require an unrealistic amount of discipline to undertake.)
Likewise, if a person's advice and their behavior/circumstances match up, it's important to evaluate whether the advice is actually effective. Did this person see this result because of the advice, or because of something else? It's possible the advice is fundamentally sound; likewise, it's possible the advice is a mistaken conclusion based on coincidence. If the person giving the advice is known to be insightful, self-examining, etc. that suggests a greater likelihood of the advice being valuable.