This issue is weaker than the rest of the Mary issue and that is why I usually don't argue it though I think that it is very unscriptural to pray to saints. There is no scriptural precedent to do so. None at all. There is direct scriptural teaching about prayer though. It is arrogant and wrong minded to go around the scriptures and look for a reason to pray to the dead as heathens have done for centuries and say that scripture allows this. It doesn't by due to lack of precedent.
But more to the point, look at the various and sundry doctrines RC's hold about Mary. The immaculate conception, assumption, distributrix of all graces and all the many things I'm sure you've seen me post. This is the much stronger case. This case is the one that makes a true bible believing Christian realize that he is involved in a system of belief that has no basis in scripture at all. This is the doctrine that shows how the RCC "uses" scripture to invent things that are in no way biblical.
Heathens have done things like pray in general for centuries. Heathens have sung hymns for centuries. Heathens have gathered for communal worship for centuries. From a narrowed Old Testament perspective, the entire New Testament is a festival of heathenry.
The fact is that devout Jews did not accept the argument that circumcision and not eating pork were irrelevancies and they did not accept the argument that worshipping a man was also worshipping God.
The inability of other Christians to receive the full implications of the Scriptural doctrine of the communion of the saints does not mean that believing the Church is eternally one in the body of Christ is a heathen notion.
The immaculate conception, assumption, distributrix of all graces and all the many things I'm sure you've seen me post.
I would certainly not argue the point that certain Catholics take saintly intercession to an extravagant and saccharine level.
The doctrine of the immaculate conception is again based on a Scriptural inference - the description of Mary as kecharitoumene or "filled with grace" implies that she was already filled with grace when the angel arrived, making her worthy to bear Christ. We know that all people are naturally conceived in sin. We know that unless we have been forgiven our sins or have never sinned we can't have grace. We know from Paul that Mary's obedient faithfulness to the law did not bring her grace. How was it possible for her to be full of grace if she was in her sins?
The inference the Catholic draws is that Christ, who "before Abraham was, I am" had preserved her from sin by his power in order to make her worthy to bear him, even under the law. The immaculate conception is a Christian answer to a Scriptural difficulty.
The assumption follows from this doctrine: if she was never touched by original sin, her body was never subject to the corruption which that sin brings. Both Enoch and Elijah were assumed into Heaven, so there is certainly Scriptural precedent.
As far as "distributrix of all graces" this is a hyperbolic statement which I do not believe to be part of official Catholic teaching. She is a participator in the redemption because God allowed her to cooperate in his work of bringing the Savior into the world. All grace comes from Christ and Christ literally came from her womb - to use a cruder Scriptural example: when Christ rubs mud on the blind man's eyes, is the mud a distributor of Christ's healing power to the blind man? In a way, yes.
But there is no doubt as to the true source of the healing power.
If Mary was conceived immaculately it is not because she was somehow sinless through her own merit - it was the power of Christ her son working in her. If she was assumed into Heaven like Elijah it was not on her own merit, but the power of her Son welcoming her into His kingdom. If she is the means He used to enter the world it is because He chose her as His vessel.
But Christ was sinless by His own divine power, He ascended into Heaven by his own divine power and any grace which anyone receives comes from Him, no matter which means He uses to distribute it.
I fully understand why you may have an argument against each one of these Scriptural inferences, but I would not believe them if I thought they were simply made up of whole cloth without regard to Scriptural truth.