I can't speak to the relative merits of flow vs. (what's the opposite? self-consciousness?), but I can testify that the vast majority of musicians practice dumb. They just recite, or jam; they don't target the hard part, or new ideas, although they might say they know they ought to -- and more cerebral work like ear-training or learning the combinatorics of music theory (mapping the possible chords and scales, and ways to combine them in serial or parallel) aren't even on the radar.
I think there is considerable value in the "dumb" repetitive mode of practicing a musical instrument. In my experience, it's pretty much the only way to commit guitar scales and shapes to muscle memory. This is particularly vital to playing fast or improvising. My guitar instructor (who was a phenomenal player) talked about playing blues licks over and over again "to get them into your fingers."
While I agree that it has its place, I'm opposed to over-reliance on muscle memory. I believe it restricts you to what you've drilled, even to the exclusion of very close neighbors.
But I'm also not trying to play extremely fast. If my goal was recital rather than composition, I would use it more. My recordings[1] are mostly total improv, free jazz, with a vocals track (later deleted) that states what chord changes are coming up, and then improvise over the first instrumental track while listening to the chord announcements.
I'm nothing more than an amateur musician but I had the chance to study with Charlie Banacos and one of the things Charlie emphasized was avoiding relying on muscle memory.
All the exercises I received from Charlie forced me to be consciously engaged in the act of playing- never outsourcing things to muscle memory.
I think that largely this was to avoid falling into mechanically playing over changes and to be actively engaged in thinking about the music at the moment it was happening and responding to it creatively.
