I think the issue is that those are humans who are dead and can't "hear" prayers.
What's the big deal though? If it makes them happy, it doesn't hurt anyone, and God doesn't say you can't do it, why should it matter?
It seems, to me, a bit hypocritical. God is the only one who can actually do anything about their problems (i.e. answer their prayers) so praying to a dead human vessel who cannot do anything about their problems, is tantamount to praying to a false god in my opinion. Erecting statues to the Mary and praying to them is the same as praying to a golden cow. They're ascribing "godly" attributes (forgiveness, comfort, etc) to a human.
Humans cannot forgive or comfort others? Besides, they aren' just a dead human vessel. They are in heaven and are Saints.
But if you're praying to a deity in heaven, then it should rightfully be god that you're praying to. Otherwise, the way I see it, you're saying that god's not the right man for that particular job, and by extension you're picking someone who you are more comfortable with to pray to. This is all well and good, but it implies that one does not have their entire faith in god. If god is perfect, there should be no need to pray to Mary and the saints. This leads me to two conclusions.
Either,
1) By praying to Mary and the saints, catholics acknowledge that god is not infallible and perfect
or
2) Catholics willingly pray to human spirits who used god's power to perform miracle (in the process elevating them to a deity status) therefore praying to false idols (after all, it was the power of god which made the miracles through human vessels, and not the humans themselves).
Either way, the notion of praying to anyone but god (and this practice being sanctioned by the Catholic church) is absurd. No?