Ron,Originally Posted by Ron St. Germain
There are several reasons why this rule applies. The main reason is that is causes the chord to become unstable. Let me explain. Let's say that you have a C7 chord (CEGBb) and you've decided that it wants to go to a tonic (we'll say F Major).
The best way to look at this is to imagine it at a cadence point. The phrase is passing along and ends with a V7-I or authentic cadence. If you write the C7 as a root chord (CEGBb) that moves to F major (FAC), the cadence is going to sound more stable. The root movement C-F (5th or 4th depending on direction) is extremely strong and sounds "final".
In common part writing rules, the root C is most likely to be doubled. It gives the chord more stability because it reinforces the root and makes it stick out.
If you have a second inversion chord (BbCEG) and you try to double the root (Bb), the plagal movement, or "Amen", is less powerful. The stability is lessened because it briefly makes the Bb the pitch center or tonic.
It leads to inproper part writing (ie. parrallel octaves, direct fifths or octaves)
You also have to take into account the context in which this was written. The russians were busy trying to please a regime that disliked unorginazation. To their ears, the incorrect doubling would sound unorganized.
In todays society, where tonality has lost it's monopoly on music, it usually sounds just fine.
I hope this helps answer your question. If you have anymore or some follow up, please feel free to let me know. Also, if anyone else has any comments, corrections or question, let me know.
Good luck and keep writing,
""Even at 6:00 in the mornin', that boy ain't right!!" - Hank Hill