musocity,
Bob (you are also Bob, right?) has a point on generative variation. No need to poke at him, just explain. Arrangers are more predictable, and they should be. You don't want "suprises" when you play live. Meaning, if one day arranger option will be a reality, styles to fit arranger would have to be edited by PG or user to sound predictable every time. To solve that, hypothetical arranger PG style could look something like a large project with tracks "frozen" so it doesn't randomize content on trigger (chord change/section).
And to replace (generative variation) of specific part, a user would have to use Partial Regeneration function to make certain instruments to play differently on specific chord(s) and that is a very manual task, considering that initial results are a randomized set. Arranger styles would have to be curated by PGmusic (picking out/generating best per chord sections), as most people interested in arranger would expect more or less flawless expierence "out of the box". So in my view that is the biggest devil in the details.