It is a good because it raises questions about hypocrisy, and Jesus was very clear that he hated hypocrisy. Why is it acceptable for this local council to add to the Nicene Creed, but it is not acceptable for another local synod (Third Council of Toledo) to also make an addition to the creed.
Firstly, the Roman Catholic Church has always held this council to be ecumenical, along with every other church. That is why it is usually called the Second Ecumenical Council. However, the Roman church does not accept canons 5 through 7 attributed to the council as these are believed to be later editions based on manuscript evidence. Indeed I myself was unaware these disputed canons existed.
Secondly, the Roman church would not refuse to accept a council intending to promulgate church-wide legislation as Ecumenical until the Council of Trullo, also known as the Quinisext Council, which is upheld only by the Eastern Orthodox, as its canons prohibit depicting Christ as a lamb* and using unleavened bread in the Eucharist (which is also the practice of the Armenian Apostolic Church) and mandates other things which are very specific to the historic practices of the Byzantine Rite, but not the other liturgical rites.
Thirdly, and perhaps most importantly, the Roman Church sanctioned participated in the Third Ecumenical Council in Ephesus and the Fourth Ecumenical Council respectively, the former being supported by Pope** St. Celestine, one of my favorite Bishops of Rome along with St. Gregory the Great, St. Sixtus, St. Clement and St. Peter, by the way, who was the main ally of Pope St. Cyril of Alexandria in his struggle against Patriarch Nestorius of Constantinople. And at the Fourth Ecumenical Council in Chalcedon Pope Leo I had a major influence as the author of the Tome, which inadvertently contributed to the tragic schism with the Oriental Orthodox (in combination with the machinations of Ibas, later discovered to secretly be a supporter of Nestorius, who tricked the council into believing Pope Dioscorus of Alexandria was still a supporter of Eutyches, and thus a Monophysite, leading to him being deposed, when in fact he had anathematized Eutyches for heresy, and the Oriental Orthodox have never actually been Oriental Orthodox).
Now, the prohibition against modifying the Niceno-Constantinopolitan Creed of 381 was adopted in the canons of the Council of Ephesus and the Council of Constantinople.
Thus, the creed adopted by the Second Council the Roman Catholic Church upheld as Ecumenical and not Local, contrary to your claims otherwise, was made immutable by the Third and Fourth Ecumenical Councils upheld by Rome. What is more, the Roman Church upheld this until the Ninth Century, when under St. Photius, the Filioque used in Toledo was introduced. Previously the Roman Church resisted adopting the Filioque despite extreme political pressure from Charlemagne to do so.
Fourthly, Rome restored communion that had been severed by St. Photius by agreeing to delete the Filioque at a council a minority of Orthodox Christians count as the Eighth or Ninth (since there are some who regard also the Quinisext Council and the Council that upheld the works of St. Gregory Palamas defending the Hesychast monks against Barlaam, who rejected the Orthodox doctrine of uncreated grace, as Ecumenical), and adhered to this position until shortly before the Great Schism of 1054.
Finally, it is worth noting that the Roman church has almost always omitted the filioque from Greek versions of the Creed, believing it would convey a heretical message in the Hellenic tongue.
As an aside, the Assyrian Church of the East never accepted the third or fourth ecumenical councils but nontheless rejects the filioque, and the Oriental Orthodox also reject it. There is also a version of
Quincunque Vult, sometimes called the Athanasian Creed, without the filioque, and this version might be original.
*I recently read, but have not verified, that the hymn Agnus Dei was added to the Mass in protest to that provision, however, the Eastern Orthodox continue to call what the Roman Catholics refer to as the Eucharistic Host the
Lamb, and I have never heard a contemporary Eastern Orthodox complaint about the hymn Agnus Dei, the target of the canon being iconographic depictions. Furthermore, the Antiochian Orthodox Western Rite Vicarate’s Divine Liturgy of St. Gregory, not to be confused with the Presanctified Liturgy of St. Gregory (which like the pre-1955 Roman Mass of the Presanctified was actually composed by Pope St. Gregory the Great), which is based on the Roman Mass, contains the hymn Agnus Dei. It is one of two Liturgies in the Western Rite Vicarate’s St. Andrew’s Service Book, along with a Divine Liturgy of St. Tikhon of Moscow, based on the Anglican liturgy modified in accordance with the recommendations of a committee he convened after being installed as Patriarch in 1917, which evaluated the Anglican liturgy for Orthodox use at the request of Anglican converts prior to St. Tikhon ‘s arrest by the Soviet
Cheka (early pre-NKVD secret police).
**Technjcally St. Celestine would be Patriarch, Archbishop, or Bishop of Rome, as at the time the title Pope was only used by the bishop of Alexandria until the mid 6th century, and it was Pope Leo I who adopted the title Pontifex Maximus, previously used by the ruling pontifex (bridge builder, meaning hierus or sacerdos, translated confusingly in most Bibles as priest despite priest being an Anglicization of Presbyter of the Roman civil religion (and once held by Gaius Julius Caesar when he was an ambitious young man starting out on the
cursus honorum).