The holding of the Miranda decision says:
(d) In the absence of other effective measures, the following
procedures to safeguard the Fifth Amendment privilege must be
observed: the person in custody must, prior to interrogation, be
clearly informed that he has the right to remain silent, and that
anything he says will be used against him in court; he must be clearly informed that he has the right to consult with a lawyer and to
have the lawyer with him during interrogation, and that, if he is
indigent, a lawyer will be appointed to represent him
The body of the opinion also says
The warning of the right to remain silent must be accompanied by the
explanation that anything said can and will be used against the
individual in court.
He must be warned prior to any questioning that he has the right to
remain silent, that anything he says can be used against him in a
court of law
When SCOTUS says do this, that is the law. Since SCOTUS say can, will, and can and will, the only safe course of action is to say can and will.
This page has some interesting discussion about the problem of the meaning of modals in a legal context. The Miranda warning does not constitute a contractual promise between the arresting officer and the arrestee, so "will" cannot be construed as a binding promise to prosecute and use the evidence in court. "Will" can only be construed as a prediction, as do "may" and "might". If the warning said "can, and mightmay be used against you", that choice or wording would suggest that the probability of statements being used against you is low – that would be totally misleading. The strongest modal should be used, because the probability is high that some statement will indeed be used against the person. Simply saying "can" suggests that it's a mere possibility, as opposed to a probability. "Can and will" is thus the appropriate construction for conveying the probability that your confession will be used against you.
"Shall" shall not be used (it is confusing since it doesn't mean what the legal profession sometimes thinks it means); "must" is just plain wrong (it's legally meaningless to say "anything you say must be used against you", when many things that a person says would be inadmissible).