The following is what I wrote in response to a remark against the Communion of Saints in the Catholic Answers Forums:
A person who believes they cannot pray to the saints simply doesn't understand the Body of Christ.
Christ is the One Mediator; we are all members of the Body of Christ. When we ask others to pray for us, they are acting within the Body of Christ. In one sense, you could say they are mediators (small "m"), in as much as they ask on our behalf, but they can only ask because of their membership in the Body of Christ. They can only work through Christ, the One Mediator. Without Christ, it doesn't work. (Paul asks others to pray for him--Col 4:3, 1 Tim 2:1, which is just before he speaks of the One Mediator...)
Let me give you a metaphor. In my work place, we have a VP that is seemingly "unapproachable". While he cares about us, the employees who do the production work, we cannot go to him to ask him of anything. Only our Director can mediate for us. Now, say there is a rule that we must keep the blinds closed in our office all day. If I don't like it, I can go straight to my Director and ask her to ask the VP to allow me to open my window and let in some light. She will do that. But if I ask my neighbor in the cube next to me to go with me to the Director, and make the same request, it has much more of an impact. Now, if no one else in the office cared about opening the blinds, or even wanted it dark, then, even if my request was granted, only my window would be allowed to let in light, instead of all the blinds being raised, and the whole office being filled with light. It would not have nearly the same impact if we all had gone to the Director and asked her to go to the VP.
Now, if I really wanted to let in the light and fill my space, I would also go to my Supervisor and have him ask the Director to mediate for us as well. He would be even more persuasive than I am because he better understands both the Director's will and the VP's will (which are one in the same since they want what is best for the company). He has already served in my position and been promoted to a position much closer to the Director (in the same way the saints who have passed on to Heaven are closer to Christ). The Director has promoted him, and put him in his position as Supervisor (as Christ has made the saints able to get to Heaven), and expects him to help serve us workers. **Note: saints are not supervisors; that's just the name used at my workplace.
The Director (Jesus) is the head of our body of workers, and we, working through her, are able to come to the VP (God).
You say that we can not and should not speak to those who have passed from this earth. What about Christ at the Transfiguration, speaking to Moses and Elijah (Matt 17:3). Jesus is a model for us, why would he converse with the "dead" while he was still on Earth if we should not do so?
Mary also conversed with the Angel Gabriel who is in Heaven (Lk 1:26-38). In Psalm 103, verse 20, David directly addresses the angels: "Bless the Lord, all you angels..." If Mary and David could do this, then surely God could allow us to speak to the saints in Heaven.
Some Protestants might say praying is always considered worship--I would ask them this: does asking God for something mean worshiping Him? Or is there much more to worship--Praise, Adoration, Thanksgiving? When my husband asks me for a snack, he is not worshiping me at all. It is simply a request.
The fourth definition of "prayer" at Dictionary.com is "to make earnest petition to (a person)"
Certainly you can have prayers that are worship. But without praise and adoration, it falls short of worship. When we pray to the saints, we mean prayer in the sense that we are asking them to intercede on our behalf. They have no power to do it on their own; they can only do it through Christ as members of His Body.
The Communion of Saints is a wonderful gift Christ has given us, and it improves our relationship with Christ, never hinders it. Through the saints, we can "let in more light".