Personally, I think there's really no doubt that hands-separate practice, while it may not be a "necessity," is useful in many ways: memorization, technical security, independence of the various parts. While it's true that in the end, the hands must be coordinated together, one of the major obstacles in playing the piano is to be able to think of each hand's part independently (e.g., play evenly, bring out voices, accents, melodic lines) while conveying the piece's overall effect. By working on the separate hand, the mind is turning all its focus to the portion that needs technical attention, not just simply playing over and over again and ingraining a problem in the reflexes. For example, in Chopin Prelude 16, there is the leaping bassline, with that initial accent that makes it so difficult, or although the black key etude is of course more difficult in the right hand, any problem with, or unnecessary amount of focus on, the lefthand leaps can make the right hand more difficult to play. Again, it's isolating the problem area and practicing it over and over again until it feels natural that often achieves the results in technically difficult music.
It goes without saying that this is dependent on the level of the individual pianist's technical attainment and the particular composer or style of the music. It would be rare that I would practice a Haydn or Mozart sonata hands separately (though slow practice certainly always helps). Chopin, on the other hand, with his often contrapuntally complex and technically difficult basslines, is one composer where, I believe, some hands-separate practice is often necessary. According to my teacher, Cortot in fact always made his students memorize the bassline in Chopin separately -- the thought being that it is in most people the bete noire. She exhorted me to do the same but alas didn't come down hard on me enough so I was often very lazy as well. But every time I have since followed this advice, I have benefitted thereby in terms of technical security. Even musically, though, the benefits can be tremendous. Practicing a single melodic line in a nocturne, for example, without the bass, can make one feel the phrasing more securely. It may help when doing so to exaggerate the gesture one wants to achieve, the thought being that in an actual performance it will "come out right" when the nerves set in. Such a modus operandi also works wonderfully when practicing slowly hands together. I emphasize slowly because it's slow practice that really ingrains reflexes in the mind and makes them sure. Few would question that Rachmaninoff is one of the very greatest pianists of all time, and it's interesting to note that no one ever heard him practice at anything other than a snail's pace.
The key point to my mind is that as pianists we're dealing, especially with the better composers, with multilayered textures. Regarding practicing the voices separately in Bach, IMO it absolutely helps, not only to get a sense of the overall structure of the entrances but to make the legato graceful and hear which ones to put in the foreground or background at various junctures, no mean feat.
Just my thoughts on a very interesting subject raised.
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