The local council of Toledo had no authoirty to make dogma. It's addition to the Creed was deemed "profitable" in combating Arian heresy and as such should have ceased when that heresy no longer existed.
A development in doctrine isn't necessarily an error, is it?
Doctrine is something the Church collectively decides in a Synod. It is certainly not the domain of a secular leader, a semi-iconoclast at that, to decide what is orthodox in the Church.
Since the filioque is not universally recited in the creed in the Church, can it be properly considered Dogma?
True. But that doesn't mean that the teaching doesn't have value or authority, does it?
It's addition to the Creed was deemed "profitable" in combating Arian heresy and as such should have ceased when that heresy no longer existed.
If it's true, why?
A development in doctrine isn't necessarily an error, is it?
Doctrine is something the Church collectively decides in a Synod.
That is the final stage. (The promulagtion of dogmas is not limited to Church Councils in the Catholic Church). But in the meantime, the teaching usually exists as part of Sacred Tradition. Such was the case with the dogma of the Assumption. Mary's Assumption had been part of Sacred Tradition prior to its promulgation as dogma.
The teaching of the double procession of the Holy Spirit goes back to the early Church:
The dogma of the double Procession of the Holy Ghost from Father and Son as one Principle is directly opposed to the error that the Holy Ghost proceeds from the Father, not from the Son. Neither dogma nor error created much difficulty during the course of the first four centuries. Macedonius and his followers, the so-called Pneumatomachi, were condemned by the local Council of Alexandria (362) and by Pope St. Damasus (378) for teaching that the Holy Ghost derives His origin from the Son alone, by creation. If the creed used by the Nestorians, which was composed probably by Theodore of Mopsuestia, and the expressions of Theodoret directed against the ninth anathema by Cyril of Alexandria, deny that the Holy Ghost derives His existence from or through the Son, they probably intend to deny only the creation of the Holy Ghost by or through the Son, inculcating at the same time His Procession from both Father and Son. At any rate, the double Procession of Holy Ghost was discussed at all in those earlier times, the controversy was restricted to the East and was of short duration. The first undoubted denial of the double Procession of the Holy Ghost we find in the seventh century among the heretics of Constantinople when St. Martin I (649-655), in his synodal writing against the Monothelites, employed the expression "Filioque." Nothing is known about the further development of this controversy; it doesnot seem to have assumed any serious proportions, as the question was not connected with the characteristic teaching of the Monothelites. In the Western church the first controversy concerning the double Procession of the Holy Ghost was conducted with the envoys of the Emperor Constantine Copronymus, in the Synod of Gentilly near Paris, held in the time of Pepin (767). The synodal Acts and other information do not seem to exist. At the beginning of nineth century, John, a Greek monk of the monastery of St. Sabas, charged the monks of Mt. Olivet with heresy, they had inserted the Filioque into the Creed. In the second half the same century, Photius the successor of the unjustly deposed Ignatius, Patriarch of Constatinople (858), denied the Procession of Holy Ghost from the Son, and opposed the insertion of the Filioque into the Constantinopolitan creed. The same position was maintained towards the end of the tenth century by the Patriarchs Sisinnius and Sergius, and about the middle of the eleventh century by the Patriarch Michael Caerularius, who renewed and completed the Greek schism. The rejection of the Filioque, or the double Procession of the Holy Ghost from the Father and Son, and the denial of the primacy of the Roman Pontiff constitute even to-day the principal errors of the Greek church. While outside the Church doubt as to the double Procession of the Holy Ghost grew into open denial, inside the Church the doctrine of the Filioque was declared to be a dogma of faith in the Fourth Lateran Council (1215), the Second Council of Lyons (1274), and the Council of Florence (1438-1445). Thus the Church proposed in a clear and authoritative form the teaching of Sacred Scripture and tradition on the Procession of the Third Person of the Holy Trinity.