Interesting thread. A couple of thoughts:
I think the fact that we inherited commentary from police driving schools is both a blessing and a curse. It's a blessing because, used wisely as a self-development tool, commentary does prompt one to lift one's vision and look further ahead and my experience over almost 20 years as an observer is that a muddled commentary does very often highlight a muddle or at least a lack of decisiveness in the driving plan, and it's good to surface that. But as civilians we do ourselves no favours if we copy some of the more "frantic" styles of police commentary, packed full of stock phrases and paragraphs recited verbatim from Roadcraft.
I can deliver a commentary in that style, but it doesn't help my driving. The risk of that style of commentary is that it becomes an end in itself and, as others have said, becomes rather disconnected from what's actually happening. I remember giving such a "full" commentary to John Lyon when I was training for HPC entry with him. After a few minutes he sighed slightly wistfully and said, "Ah, James, if only your driving was as good as your commentary says it is we'd be getting somewhere."
If I think about the people whose commentary I really admire, the characteristic they share is that their commentary is quite spare. They use a few words well and what they say consistently prioritises the most important issue in view at the time, which reflects the fact that they both have very highly developed planning skills. They are not trying to impress me with their commentary. In fact they're not trying to impress me at all, because their driving is markedly free of ego. Stefan Einz posted some commentary video on this forum which I can't currently find by searching for it, but that exemplifies the style that I admire and think works well.
Kimosabe's hint about commentary being a tool for suggestion is interesting. I use it myself that way sometimes. One of my long-standing issues is that if I turn from a major road to a minor I'll find my driving drops in quality, pace and "sparkle" for a bit, I'm slow to acclimatise to the new road. So I've borrowed a trick from a police driver friend and now say to myself out loud as I turn into the new road "New road, vision up..." as a reminder to raise my vision and really look at the new scene in front of me.
When all's said and done, commentary is a slave not a master. Some will find it useful, some won't. The danger of distraction and becoming overly absorbed in it is very real. Police officers, of course, sometimes have to give evidential driving commentary during a pursuit and it's not insignificant that in most forces, the standard is now that if the pursuing car contains two officers, the driver drives - and concentrates on that - and the co-driver gives the commentary. Similarly, in the driver training program for the London Air Ambulance rapid response cars (which are driven by an ambulance paramedic but might have to travel right across London to reach a call when the helicopter can't fly) the standard is that all the passenger does is to give route directions. Silence is maintained as far as possible to allow the driver to concentrate. Nor, generally, is the driver allowed to know the nature of the call they're driving to, so as not to add pressure to them.
James
Only two things matter: attitude & entry speeds.