Have you ever asked a family member or friend from your ecclesial community to pray for you? If so, why? Why don't you pray to God instead?
My belief in the communion of the saints is based on taking Our Lord at His word when He said that God is the God of the living, not the dead. The "dead" in Heaven are more alive than we are. So at least with Catholics (I won't speak for anybody else), we don't believe that mortality is a barrier in asking other Christians to pray for us.
EDIT- As a guess, I'm thinking someone will eventually mention "necromancy". Knowing that my forewarning here in this post will ultimately be all for nothing, I still suggest that whoever is inclined to bring up necromancy should well understand the means, the nature and the purpose of necromancy. Things that are different are not the same. Understand what necromancy is before bringing it up.