I for one would never argue that its important to get oil in "nooks and crannies."
I would say that its important to keep oil flow volume high to carry away heat, flood the non-pressure lubed areas like rocker tips and rocker fulcrums in Chevy/Ford/AMC style stud-rocker designs or the pushrod tips in Mopar/Olds/Cadillac style shaft rocker engines), drench the undersides of pistons, drain back to the pan faster than the pump can drain the pan, etc.
I'm not a "thin oil guy" per se (I run 5w40 in my Jeep which is spec'd for 10w30 for example). But since film strength doesn't 100% correlate with viscosity, I don't believe that running super thick oil does any good either, and its been my experience that beyond a certain point oil consumption increases with thick oils (presumably because the oil control rings get overwhelmed). I would hazard a guess that at the kinds of pressures seen between a lifter face and a cam lobe face, or even between a bearing shell and journal on the power stroke, the difference in thickness between 20 wt and 50 wt is trivial. Its not the thickness of the oil that's making it work in that regime.