if you manage to shadow someone for many months,
and he gives you a very strong letter, do you think it would matter much at all?
Very modest. Would not recommend generally.
You'll hear people disparage anything that is off the beaten path.
I matched this year with 3 academic letters and a private practice letter. The private practice letter was phenomenal and I received many compliments on it even though most didn't know who the nsg was. I had big name letters as well and it was a good mix to please the faculty members who only wanted to see big names they recognized but also was good for the people who wanted to see a well written letter that was clearly not a "copy paste" even if it was from a lesser known neurosurgeon.
In the end, I think it better be a damned good letter, otherwise stick to the academics.
It is probably not important