Actually, Divine simplicity is a key doctrine with astounding implications. It's not just a revealed doctrine but a key tenet of natural theology, following directly from God as ipsum esse subsistens.
Some have argued against it on the basis of modal collapse. While it's true that modal collapse objections don't work as an argument against the doctrine (contra Bill Vallicella), what's important is why they don't work. Let's give the following modal collapse argument which is certainly valid and cannot be objected to by reason of substitution in referentially opaque modal contexts (since necessity is the only modality we are using).
1. Necessarily, God exists.
2. Necessarily, God's essence is identical to His existence.
3. Necessarily, an act of creation by God (referring to what is intrinsic to God) is identical to the act of God.
4. Necessarily, the act of God is identical to the existence of God.
5. Therefore, necessarily, God's act of creation is identical to the existence of God.
6. Therefore, necessarily, God's act of creation is identical to the essence of God.
7. Therefore, God's act of creation is necessary.
8. Necessarily, God's act of creation entails what is created.
9. Therefore, what is created necessarily exists.
10. Necessarily, everything that exists that is not God is created.
11. Therefore, everything that exists exists necessarily.
Only one possible objection to validity could be made and that would be to say that "an act of creation by God" in 2. is not a strictly rigid designator - it does not refer to the exact same thing across all possible worlds. However, this objection is shown to be begging the question - conclusion 6. shows that it is in fact the same thing across all possible worlds, since God's essence is the same across all possible worlds, and the act of creation by God is identical to it in all possible worlds.
Now, since the argument is valid, but the conclusion is incorrect (modal collapse), one of the six premises (1, 2, 3, 4, 8, or 10) must be incorrect.
1. is demanded by Divine aseity.
2. is demanded by Divine simplicity (as well as aseity).
3. is demanded by Divine simplicity (there cannot be a multiplicity of acts in God).
4. is demanded by Divine simplicity (there is no distinction between what God is and what He does).
10. is demanded by God as First Cause (everything that is not God is caused to exist by Him).
The only way to avoid the conclusion is to deny 8. Which means that there is no pre-determination by God in the created universe, not even by way of "from eternity" - God's creation is non-determinative. (A similar argument holds if, instead of creation, other objects of God's willing are used, so we can conclude His causation is non-determinative.) To say the universe exists "because" God created it only means that the universe is ontologically dependent on God for its existence. It does not mean God pre-determined that this universe should be the one which exists, or that any particular thing in it happens. But, couldn't at least some things (even if not all) be entailed by God's act of creation? No, because any of those things would likewise exist necessarily, meaning God creates them in all possible worlds. Put simply, "God wills X to exist" has no real informational content beyond "God exists, and X exists".
Thomism would like to have it both ways (Divine simplicity and Divine pre-determination) but without success. It adds something extrinsic to "God's act of willing", the object of God's will (X, say), and says that while God wills His own goodness necessarily, He only wills X contingently as a means of willing of His goodness, as willing X is not necessary for that end. But that single act of will is identical to the existence of God and therefore can't predict whether X will or will not be willed and hence whether X will or will not exist anymore than the existence of God can. There is nothing "about" God's will, considered intrinsically and in itself, that distinguishes between willing or not willing X (this is different from created wills, where willing X in particular is an accident). What it actually means for X to be the "object of God's will" is quite murky. Thomism falls back on X being "in the mind of God" before it is actually created but that is making it something intrinsic to God. Or, if "in the mind of God" is not something intrinsic, then what is it then? Anyway, if something extrinsic to God entails that He wills X, then He is ontologically dependent on that something and not a se.
We naively think there this is no conflict between Divine simplicity and Divine pre-determination because in created beings like ourselves existence and will are two different things. That is not the case with God; there is no real distinction between the two but only a notional distinction. Our wills are only analogously the same to God's will; they resemble it in some way such that it is not incorrect to use the same term and it is not equivocal (as in "The bank is closed" vs. "Walking on the west bank of the river").
Granted this has profound implications, but let's start with this in itself.