Police should always have to read someone their miranda rights, otherwise cops could much more easily abuse someones rights, which a lot of cops, lets face it, would like to do. Police officers are always look for an excuse to abuse someones rights, so we can't let them do it any further.
The Constitution presumes that all people accused of a crime are innocent of that crime unless proven guilty beyond a reasonable doubt. This is a fundamental pillar of a free society. Thus, the government bears the burden of proving one's guilt. One should not be compelled to incriminate himself, so the right to remain silent is a vital protection of liberty, and police must let defendants know of that right.