To sum it up
The filioque can mean two things:
1) That the Son sends the Spirit into the world, just like the Spirit sent the Son;
2) That the Father and Son are not different in anything except being Father and Son and therefore the Spirit proceeds from both at the same time eternally.
Meaning (2) is the one implied by the 'filioque' in the context of the Creed text. Because Orthodox Catholic tradition holds (1) to be true and that (2) is false, and because the 'filioque' in the Creed can only mean (2), then the Church, along with Pope Leo III and his predecessors, refuse the insertion of the filioque in the Creed imposed by Charlesmagne as both blasphemous and illegitimate.
It is illegitimate because no king, emperor or local synod can impose dogma on the Church or alter the Symbol of Faith, specially in disregard of both the synods and the primate. In analogy to secular government, it's like a state, governor or the president changing the constitution without consultation to the congress.
It is blasphemous because:
a) it disregards and alters the direct words of our Lord Jesus Christ:
But when the Comforter is come, whom I will send unto you from the Father, {even} the Spirit of truth, which proceedeth from the Father, he shall testify of me. St. John 16:26
; Our Lord is the very author of the distinction of "proceeding of the Father" which must not include "and from the Son" and the sending of the Spirit by the Son to the word. If these things could be expressed with "filioque", the Son Himself would not have made the distinction in the sentence above;
b) Despite traditional Catholic theology that there are only two attributes in God, those that pertain to the Godhead and those that are exclusive of each person alone, it creates a third category of a trait that is shared by two and not by one;
c) With the creation of this third category, it reduced the distinction of the Father and the Son to mere words with no
de facto difference than the use of different words. If it were true that there is no distinction between the Father and the Son except that one is the Father and the other is the Son, then
Patripassianists would be right (
http://en.wikipedia.org/wiki/Patripassianism).