Sometimes it seems more people are about witch hunts than figuring out the real truth. Sometimes it does matter what people say because of this. People just know how to turn other people's words against them. You can always make an argument against anything. Many people know this, and figure it is better just to let the evidence speak for itself. Other people clearly do it because there is nothing they can say to get them out of what they did.
No they aren't always guilty of a crime. Some just do it because they are trying to protect another person from getting into trouble or they are trying to hide the truth because they are being or feeling threatened by someone else that is around them and they know that person might come after them.
No, I do not think that people who publicly exercise the 5th Amendment are always guilty of a crime, because they know that the government will use anything against you. Law enforcement often just wants a conviction, regardless of the truth. People are smart to safeguard their rights in the face of dishonest cops.
I do not believe people who publicly exercise the fifth amendment are always guilty of a crime. Everyone should exercise this right when being arrested, regardless of the circumstances. When you are being questioned or arrested, law enforcement is trying to get you to say incriminating things. This can be a manipulative process and it should be avoided. It is better to plead the fifth and speak with a lawyer before you say one work to law enforcement agents.