Here are the arguments and the original text by Diether de la Motte and a link to his book:
.... parallelisms that actually occur in the music of the great composers.
There are two usual explanations for the prohibition of parallel perfect octaves and fifths between any two voices:
1) The octave has a simple ratio of 1:2 between the frequencies of its lower and upper notes (for example: if the frequency of the lower pitch is 100 Hz., then the frequency of the upper pitch will be 200 Hz.). Because of this acoustical phenomenon, the two tones of the octave are often perceived as a single unified sonority. Similarly, the perfect fifth, which has thenext most simple ratio (2:3) between its upper and lower tones, is also frequently perceived to be a strong unified sonority. Therefore, two voices which move in parallel octaves or perfect fifths will tend to lose some of their own melodic individuality and independence.
*2) Parallel perfect octaves and perfect fifths simply sound "bad."
Both of these traditional explanations are somewhat troublesome. In explanation 1), why are we not also cautioned against the use of parallel perfect fourths the next most simple ratio (3:4)?
and De la Motte argues:
In explanation 2), why is it claimed that parallel perfect octaves and fifths sound "bad," when, as shown in Ex. 1:26, such structures abound in the music of the Middle Ages? (Furthermore, in the style of Medieval parallel organum, the parallelisms in question are said to have a particularly "good" sound.) Example 1:26 Excerpt from a Thirteenth-Century Conductus * Parallel perfect octaves and fifths, broken by rests or cadences, were also generally pemitted and cmployed im later musical styles.
All technical arguments which attempt to explain why parallel perfect octaves and fifths should be avoided create problems that cannot be resolved. For that reason, I shall attempt to explain the origin of this rule a different way. The major triad is a basic harmonic element that was common to Western music during a period of some 500 years (from Dufay to Reger). It also plays a vital role in twentieth-century music, especiallyinthe works of Hindemith and Stravinsky. Dominant-seventh and diminished- seventh chords, which are newer structures than the major triad, are quite rare in twentieth- century music literature, however. This is because both of these sonorities are associated with the kind of dominant-function harmony that is particuarly characteristic of the Classical and Romantic periods. By 1925, many composers refused to employ dominant- or diminished- seventh chords in their works because these sonorities evoked musical styles and aesthetics that were no longer in vogue. In short, harmonies which had played such an important part in the music of the past were simply rejected as old-fashioned by composers of a newer age. Parallel perfect octaves, unisons, and perfect fifths were first considered errors of composi- tional technique in the fourteenth century.
In many ways, compositions of this era also manifest
an aversion to earlier musical expressions.
Since composers of the fourteenth century considered the music ofthe previous era to be primitive, it is possible that disdain for and avoidance of earlier techniques helped establish the rule that: parallel perfect consonances are musically incorrect.
Composers always double different parts in octaves, NEVER in fifth, even though both intervals destroy voice independence.
There MUST be some other reasons why parallel fifths are banned. Could anyone suggest other reasons?
Well, as you see, your question provoked some good answers, but eventually not in the sense of your last point!
As now rereading your question I have to say: Never say never!