Slightly rant-y about religion below, but I hope it's more informative than inflamatory.
Plenty of Christians (like, all Protestants) believe Saints are unnecessary/irrelevant/nonexistent, but their clearly subordinate status makes them different from polytheism AFAICS.
I think you underestimate how much Islam is REALLY, VERY concerned with not being ANYTHING like ANY SORT OF polytheism. It can be said to be THE central theme of the Qur'an. There are others, but the Oneness and Completeness of One True God is repeated, reinforced, and driven home time and time again.
The opening sura of the Qur'an is pretty much concerned with devoting oneself to God and God alone and no other God and staying on the Straight and Narrow. Compared to, say, the book of Gensis, which is concerned with obeying God and listening to God and not defying God....the fall of Mankind for most Christians is disobedience. The fall of Mankind for most Muslims is Polytheism. It is a grave sin in Christianity to defy God's commands. It is a grave sin in Islam to obey any commands BUT God's. Jesus kicks the moneychangers out of the temple because they were defiling faith. Muhammad destroys the false idols of the Ka'ba because it is polytheism.
I'm not saying that Christians are somehow more permissive of false idols, but I am saying that one of the central
jihads in Islam is the struggle against people who do not worship the True God; polytheists. Polytheism (or accusations thereof) have been the main reason that other versions of Islam have split from Sunni. A polytheist is one of the worst things a Muslim can be, because that is to not be a Muslim at all.
There is no such thing, in most Islam, as an intermediary between Mankind and God. No lower rungs that you can pray to. No more sympathetic ears that have powers that may help you. There is God, and God alone and no other supernatural being other than Him and what He has created, and he has not ever created anything that should be prayed to.
Yes, the theory is that Saints get their power from God and this aren't the same as Polytheism. Many Muslims would say that argument doesn't hold water. Every time you pray to a saint instead of praying to God, you are worshiping a false idol. Why not just pray to God?
Again, this isn't meant to really be an inflamatory post, just sort of reinforcing the idea that most Muslims are usually VERY hardcore on the whole propper-attribution-of-worship angle. So if he's nervous about polytheistic tendanceis, making subordinates grant spells won't really help the situation much.
That said, RPGs are in an imaginairy noplace where the usual laws of everything are entirely nonexistent, and the only laws are set down by the DM, a human being, and have no relation at all to the true laws governing the real world, whatever one believes them to be. It is inherently false, and as long as everyone knows it's inherently false, it's just playing with imagination, story, and fiction. It reflects reality no more than Tolkein does, which is to say not at all.