There is clearly a theological defense for it; if you can ask for the prayers of the living, why should you not ask for the prayers of the dead? There is clearly also a great danger. In some popular practice we see it leading off into an infinitely silly picture of Heaven as an earthly court where applicants will be wise to pull the right wires, discover the best “channels,” and attach themselves to the most influential pressure groups. But I have nothing to do with all this. I am not thinking of adopting the practice myself; and who am I to judge the practice of others?
The consoling thing is that while Christendom is divided about the rationality, and even the lawfulness, of praying to the saints, we are all agreed about praying with them. “With angels and archangels and all the company of heaven.” ... One always accepted this with theoretically. But it is quite different when one brings it into consciousness at an appropriate moment and wills the association of one’s own little twitter with the voices of the great saints and (we hope) of our own dear dead. They may drown some of its uglier qualities and set off any tiny value it has.
You may say that the distinction between the communion of the saints as I find it in that act and full-fledged prayer to saints is not, after all, very great.
C.S. Lewis, Letters to Malcolm 15-16.