in most places your packing in, weather can come in -unexpected, anytime. bivy sacks are light and the first time you get "caught " with out something, you'll wish you had brought at least a bivy sack.
i used to sleep under the stars. one time a snow storm, THAT WASN"T CALLED FOR, came in and i got soaked that night. woke up at 2:00 am and built a fire out of a pitch stump, and as soon as it broke daylight i had to get the heck out of there. that sucked- but it wont happen again.