Its optional on most. Most dealers probably don't order it because most shoppers wouldn't think of that while shipping.
Roadside assistance will rarely bring a spare to you, unless they are affiliated with a tire dealer. Most likely you would be out of luck in the middle of nowhere. At the vary least, carry an un-mounted tire because more roadside assistance companies can mount a tire in the field. Most commercial truckers do this so they don't have to lug a rim around. But this still leaves you in a situation where you would have to call for a truck, which may not always be an option.
