I'm aware that 737-800, vnav shouldn't be armed for takeoff with anything less than U10.8A... but thanks nonetheless.
The best explanation I can come up for myself is due to the fact our tech crew operate both the 400/800.
I think the practice of not arming vnav for takeoff, in an aircraft that has the capability for it to be armed, stems from the 400 (or classics in general).
I think the procedures have been designed to be uniformed; that is, an appropriately endorsed pilot can jump from the 400 to the 800 <and not arm vnav for takeoff>.
I'm just having trouble finding the link, and confirming my theory.