We're discussing two very different concepts here. You are alluding to divination; catholics believe in the intercession of the Communion of Saints.
In praying to the saints, one is not seeking knowledge about the future, but rather asking for their prayers. James 5:16 states: "The prayer of a righteous man is powerful and effective." The saints in heaven are not dead; they are more alive than we are. Scripture never says that those who die in the Lord are "dead." If we can ask one another to pray for each other, why can we not ask those already in the presence of God to pray for us? Is this not what Revelations 5:8 speaks of when it tells us that the 24 elders offer the bowls of incense that are the prayers of the saints? Also, if "communicating" is condemned, why does Jesus communicate with Moses at the Mount of Transfiguration in Luke 9? Deuteronomy 34:5 tells us explicitly that Moses died. Hebrews 12:1 speaks of a great cloud of witnesses. Scripture tells us that Christ is the vine and we are the branches. I know of no Scripture that says that earthly death separates us from that vine.
People with familiar spirits don't practice divination. They just speak to the familiar. Divination would be one form of communication associated with necromancy; but, only one. Divination in that sense would be the fraud of thinking oneself able to call up the dead when all they are actually doing is conferring with a familiar spirit - a demon. That will get you in trouble in a heartbeat - as in a death sentence under the OT rules.
The dead cannot interract with the living. A principle dealt with not only in the old testament; but in the new. Some would say, 'yeah; but, Christ raised people from the dead.' Read your old testament and you'll find people were raised from the dead by the power of God before Christ was born. The dead can't interract with you. God forbade it. But if you try to communicate with the dead, the demons are happy to oblige in making a good show of it. You can't see into the spirit world, so all they have to do is the minimum of familiar discussion and you are decieved. Thus the term familiar spirit. Once you do this, you give license to Satan in your life by putting your trust in the image he has represented to you - just as Eve did in the Garden. But I'm sure you think yourself wiser than her. That would be expected - all men think that way till God gets hold of them. That's why Solomon told us that there is a way that "SEEMS" right to a man but its end is DESTRUCTION. Satan made it seem right to EVE that eating the fruit was ok. It's end was destruction - death entered the world, the order of things was upset. The spiritual and the physical seperated. One sin did that. Adam didn't have to follow suit, Eve did all the damage that was needed to be done. One sin. One rebellion against God by someone who decided something else through rationalization seemed ok despite what God said.
In praying to the saints, one is not seeking knowledge about the future, but rather asking for their prayers.
Saul sought aid from Samual. No different than asking the saint of dog bites (yes you have one and I'm not joking) to take care of the bite. Seeking aid is seeking aid, no matter how you try to make it seem otherwise. One has to communicate in order to seek aid. And Communication is the thing that is disallowed no matter whether you seek aid or not. No matter what kind of communication it is, it is still communicating with the dead. Forbidden in Ecclesiastes 9 and outlawed with death penalty under the law in every form it takes, whether it be necromancy, spiritism, consulting a familiar (the default form), ect. Understand, that since God has ruled that the dead can't communicate or interact with us, that doesn't stop demons from doing it in their place. That is why the default form is consulting familiar spirits. Demons will answer and play act for you at being your dead grandma. You can't see them, and all they have to know is a few things grandma had in the house, something she might have said, ect. then you're hooked and counseling with a demon. For which God would have you put to death. It isn't yours to reason why. You are supposed to be obedient even if you don't understand why. Not to me; but, to God. He's the one you answer to. You can sit and make excuses to me; but, when Dad gets home...
"The prayer of a righteous man is powerful and effective."
Not untrue. It was still unlawful for Saul to talk to Samual. You haven't dealt with that. You are applying things said of the living to the dead as though this is ok to do. It isn't. It's bad exegesis and it is begging reason instead of founding in scripture. I can argue that camels are wise counsel with reason. It don't make it either true or scriptural. Let's say it's illegal to park your car in front of the courthouse. Parking a car is a wise thing. Nice cars are even better. Nice cars with good gas mileage are better yet. A nice car with good gas mileage that was impervious to decay would be the best. But if I park that nice impervious car with good gas mileage in front of the courthouse, it's still illegal. You're describing the car to argue your right to park in front of the courthouse knowing it's illegal. If I'm smart enough to see it, how in the world do you believe you are ever gonna Con God with that.