"I've never seen "My documents" folder getting deleted. Nor has ever anyone complained about that." -> https://www.google.com/search?hl=en&safe=off&q=windows+update+delete+%22My+documents%22
"For existing sites, you can mass-modify them:" -> Because it makes more sense to command line to modify every single site than having a default setting for a local folder...
"For existing sites, you can mass-modify them:" -> Because it makes more sense to command line to modify every single site than having a default setting for a local folder...