The attorney-client privilege covers communication made by the client to the attorney for the purpose of the legal representation. It also includes the attorney’s communication to the client based on confidential information from the client, and there are exceptions like if it wasn’t made with a reasonable expectation of privacy. How useful it is to the prosecution isn’t a factor–there’s not a balancing of competing interests.
Note that this doesn’t mean your attorney can never be called to testify. For example if your attorney witnesses you committing the crime, that isn’t privileged; unlike with you confessing the crime to him, there’s no *communication*. Attorneys have an ethical responsibility not to represent clients in cases they are likely to be called as a witness on contested issues.
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