I don't have much of a problem. Heck, nearly every job seems to have 'complete' delayed by a window here and a door there. A little rain won't bother the Romex. Period. Just think of all the other 'indoor' materials that are OK for temporary, in the weather, on-site exposure (OSB, etc).
More of a concern is theft and vandalism; that's the real reason most EC's wait until the site can be 'secured.'
Thermal insulation is another matter. I suspect that most places schedule the 'rough' wiring just before they complete the 'dry in;' they want the sparkies out of the insulator's way, and the sparkies prefer to not have to work around piles of drywall.
