What makes you so sure that every defendant is guily? What makes you think that all people accused of a crime have committed it? Isn't everyone still considered innocent until proven guilty?
Over the long run, our judical system has proven to be fair and effective, a few aberrations notwithstanding.
Yes, they are. But I believe if they are innocent, the right to remain silent does nothing to advance the objective of discovering the truth. The same applies to conversations with their attorneys.