" A balaclava is one of the warmest items per oz. to carry."
it is exactly as Buck says.
all i can add to that is when you are toting a not-so-ultralight load, and the days starts out nippy, it's all about "oh, today we should layer" which is fine, until you get warm'ish, and need to un-layer, and then it's time to take off the heavy pack, adjust layers, put on the heavy pack (it can hurt) and go another bit, only to do it all over again. (heck of a sentence right thar).
but, if thee has a balaclava on to start with, then with a simply yank, you can make a nice adjustment to temperature without taking off the pack. it stores in your front bag or pocket. easy as pie.
for those nasty "in the shade it's cold, and in the sun it's hot" situations, a bali is just the ticket.
and all this goodness is, as Bucks says, for 1 oz.