"And when he had taken the book, the four beasts and four [and] twenty elders fell down before the Lamb, having every one of them harps, and golden vials full of odours, which are the prayers of saints." -Revelations 5:8
"The witnesses who have preceded us into the kingdom, [Cf. Heb 12:1 .] especially those whom the Church recognizes as saints, share in the living tradition of prayer by the example of their lives, the transmission of their writings, and their prayer today. They contemplate God, praise him and constantly care for those whom they have left on earth. When they entered into the joy of their Master, they were 'put in charge of many things.' [Cf. Mt 25:21.] Their intercession is their most exalted service to God's plan. We can and should ask them to intercede for us and for the whole world." CCC 2683
It's really quite simple. Those who have died and gone before us, hear our pleas and intercede on our behalf. Why not go directly to the Father? We do! But it is always good to have some allies in Heaven. In order for someone to be declared a saint, two miracles must be attributed to them. The Friars of the Atonement are praying to God and also asking Archbishop Sheen to 'intercede' on behalf of Terri Schiavo. Such a miracle would be credited towards his canonization. Got it?
"Those who have died and gone before us, hear our pleas and intercede on our behalf"
There is no scriptural basis for this statement. Hebrews 12:1 states that we are surrounded by a great company of witnesses but it does not say that they hear our pleas or intercede on our behalf. By making this statement you are adding to the word of God.
Wherefore he is able also to save them to the uttermost that come unto God by him, seeing he ever liveth to make intercession for them.
He - Christ ever liveth to make intercessession for us.
"But it is always good to have some allies in Heaven."
We need only one "ally" - Jesus himself
For there is one God, and one mediator between God and men, the man Christ Jesus;
One mediator - not many. If you think you need "some allies" other than Christ you minimize his great sacrifice.