Not only that but Samuel was pretty ticked off about being disturbed - NOT the kind of attitude expected of one who is "supposed" to be available 24/7 for "intercession". Also, Saul lost his kingdom and, eventually, his life for this disobedient act. Going to the dead was against God's command - one He NEVER rescinded.
Saul was using a witch to communicate with a spirit. This is quite different than asking someone to pray to God for you. The point remains that the dead can hear our prayers.