Very interesting post! thanks....
Classical for me is like Tai-Chi....Jazz is like real martial art....
In Indian music the mix between improvisation and planning is balanced , it is why this style is so deep....
None is inferior though in any musical styles......
Each musician is a personnality and the life of a classical musician is not the same life than a jazz one in any written novel....
Chet Baker or Miles Davis dont look like the usual classical musician ....
How Baker or Miles did their practice? They play all night in a bar drinking ....
How did Murray Perahia did his practice? Alone in a room....
And between our two hemispheres one goes looking his internal map, the other one wait for the unrecognized around him....
Any musician can chose to be anything he want , he is free but habits are also masters....
We are programmed by ourselves...
This is at the same time our freedom and the limit of our freedom...
Our brain obey our heart, but when the heart has set his choice the brain take the habit seriously....
Any jazz musician can play jazz, or the reverse, but they win usually at one game or the other....Keith Jarrett being an exception....
But remember like said frogman that the greatest musicians were master at improvisation....The crowds came to see them improvising and were amazed... For example Bach walking kilometers to listen Buxtehude improvising, or Bruckner so amazing at organ improvisation that people dumbfounded claimed that he was Bach reincarnated....Bruckner was born the greatest organ improviser before learning how to compose... Remember Paganini mesmerizing audience in a trance by violin improvisation so flabbergasting people thought he was possessed by the devil.... Listz and Scriabin after him founded modern piano music by improvisation on the spot also.....
For example the Vivaldi phrasing style is so remarkable, that we know right on the spot that this priest with a special permission from the pope to interrupt a mass to play music, improvized so easily, that he said that he was "able to compose more speedily than for someone to transcribe anything written "....
Reality is a complex matter, the brain, too and the heart only is simple because he is the master of his two slaves hemispheric servants...
Musicians improvise or play a written score because their heart said so at the times they chose....