Since the question of the normative force of the Creeds have arisen, I thought it would be useful to discuss a little Aquinas's own take on the purpose of creeds and how he handles the creeds. His attitude and handling I think would prove rather illuminating as to how we should approach the creeds.
The relevant passage can be found in his Summa Theologica 2.2. Q1. Article 9. "Whether it is suitable for the articles of faith to be embodied in a symbol?"
Objection 1. It would seem that it is unsuitable for the articles of faith to be embodied in a symbol. Because Holy Writ is the rule of faith, to which no addition or subtraction can lawfully be made, since it is written (Deuteronomy 4:2): "You shall not add to the word that I speak to you, neither shall you take away from it." Therefore it was unlawful to make a symbol as a rule of faith, after the Holy Writ had once been published.
Objection 2. Further, according to the Apostle (Ephesians 4:5) there is but "one faith." Now the symbol is a profession of faith. Therefore it is not fitting that there should be more than one symbol.
His answer to these particular objection is of vital interest:
I answer that, As the Apostle says (Hebrews 11:6), "he that cometh to God, must believe that He is." Now a man cannot believe, unless the truth be proposed to him that he may believe it. Hence the need for the truth of faith to be collected together, so that it might the more easily be proposed to all, lest anyone might stray from the truth through ignorance of the faith. It is from its being a collection of maxims of faith that the symbol [The Greek symballein] takes its name.
Reply to Objection 1. The truth of faith is contained in Holy Writ, diffusely, under various modes of expression, and sometimes obscurely, so that, in order to gather the truth of faith from Holy Writ, one needs long study and practice, which are unattainable by all those who require to know the truth of faith, many of whom have no time for study, being busy with other affairs. And so it was necessary to gather together a clear summary from the sayings of Holy Writ, to be proposed to the belief of all. This indeed was no addition to Holy Writ, but something taken from it.
Reply to Objection 2. The same doctrine of faith is taught in all the symbols. Nevertheless, the people need more careful instruction about the truth of faith, when errors arise, lest the faith of simple-minded persons be corrupted by heretics. It was this that gave rise to the necessity of formulating several symbols, which nowise differ from one another, save that on account of the obstinacy of heretics, one contains more explicitly what another contains implicitly.
As such, the reason why creeds exist is a pragmatic one, so that "it might the more easily be proposed to all". He further expounds this point in his explanation of Objection 1 when he discusses how most people don't have the time to perform long study of the Scriptures, as such, a creed summarises Scriptural points or the "articles of faith"/"rule of faith" of Scripture for ease of access for the layman. Aquinas affirms that the creeds add nothing to the Scripture, properly the rule of faith, but they merely summarise Scripture points for the ease of the layman.
His answer to Objection 2 further illuminates the purpose of creeds, the reason why multiple creeds exist is as an answer to specific errors which arise. Creeds exist as such to answer those specific delimited errors as and when they arise in the course of the history of the Church.
However, what happens when the creeds themselves employ obscure terms which the layman no longer understand? Or where instead of summarising the contents of Scripture for ease of lay reference they themselves confuse the layman? (As we can see over the debates as to what "person" means.) A symbol which is meant to be a summary of Scripture obviously cannot perform that function when the symbol itself needs further explanation and long study to be understood. And if the specific errors to which they were framed has since long disappeared (who today argues the Arian point that there was a time when Christ was not? Or the Homoian point that Christ is merely like to the Father?), then what would be the point of the creed?
To answer this point I think it would be useful to observe a curious inconsistency in Aquinas in this Article. He gives the following general answer which seems to imply a general inerrancy concerning the creeds:
On the contrary, The universal Church cannot err, since she is governed by the Holy Ghost, Who is the Spirit of truth: for such was Our Lord's promise to His disciples (John 16:13): "When He, the Spirit of truth, is come, He will teach you all truth." Now the symbol is published by the authority of the universal Church. Therefore it contains nothing defective.
Whether we Protestants can believe this, Aquinas later gives a reply to an objection which seems to qualify this claim, or at least treat it with some irony:
Objection 5. Further, Augustine (Tract. xxix in Joan.) expounding the passage, "You believe in God, believe also in Me" (John 14:1) says: "We believe Peter or Paul, but we speak only of believing 'in' God." Since then the Catholic Church is merely a created being, it seems unfitting to say: "In the One, Holy, Catholic and Apostolic Church."
[...]
Reply to Objection 5. If we say: "'In' the holy Catholic Church," this must be taken as verified in so far as our faith is directed to the Holy Ghost, Who sanctifies the Church; so that the sense is: "I believe in the Holy Ghost sanctifying the Church." But it is better and more in keeping with the common use, to omit the 'in,' and say simply, "the holy Catholic Church," as Pope Leo [Rufinus, Comm. in Sym. Apost.] observes.
Aquinas here incredibly concedes the basic objection: it would be unfitting to follow the letter of the creed to say that we believe "in" the Catholic Church, and that it would be better to just drop the "in" as has happened in common use. However to save the creed, he interpolates the creed by saying that it "really" means "I believe in the Holy Ghost who sanctifies the Church", not in the Church per se.
Thus despite what Aquinas claims about the infallibility of the Church and the lack of defects in creeds, he's not opposed to interpolating the creeds or outright simply dropping entire terms altogether if it were unfitting. Aquinas, had he engaged the prior creed of 325, would doubtless have approved of dropping the entire clause of the Son being begotten "from the substance of the Father".
As such, to return to the title of this post, are we bound to the letter of the creed? The structure of Aquinas argument suggests no. The creed exists primarily for a practical purpose: (1) for ease of reference for the layman of Scriptural articles of faith, (2) to answer specific heresies and problems. However, the moment the creeds are misleading, no longer summarise but confuse people as to the meaning of the Scriptures, he is happy to simply drop terms from the creeds. We are as such not bound to the letter of the creeds which value is justified by its function. The moment it ceases to perform its function, we may freely discard it.
Therefore to conclude, the creeds do not exist for themselves, they exist for a specific purpose, to summarise the articles of faith in Scripture and to answer specific heresies. The moment however the creed cease to perform this function, we can either attempt to reinterpret the creeds, as Aquinas has done, or we can just drop the terms altogether.