My trekking poles are absolutely essential for me--my balance is not 100% anymore--and have saved me from a number of potentially serious falls. I certainly won't venture out on the trail without them. They also hold up my tent, saving the weight of a tent pole. I'd call them true multiple-use items. The splint/travois emergency functions I mentioned in the other "multi-use" thread could, of course, use sticks instead, unless one is above timberline.

The pliers on my Leatherman Micra serve as a pot lifter, so that's a use other than the normal uses for a knife. I don't have to carry a potlifter or a potholder (I have a habit of burning up the latter, even at home). I also find the pliers useful for threading straps through buckles--they have been known to pull out of the buckles occasionally and I find grasping 1/16" of a strap end with my fingers almost impossible. In fact, the pliers are the item on the multitool that I use the most.

My silnylon (non-breathable) rain gear serves as a vapor barrier layer in my sleeping bag on below-freezing nights. I would agree that using it for wind (or horsefly) protection is a normal use for rain gear and not multiple use. It of course would be no use as a vapor barrier if it were breathable.

While the frisbee listed in the other "multiple use" thread certainly has the most multiple uses, it is definitely not an essential item!

Thinking about "mutiple use" does lead one to considering various innovations one could use existing gear for in an emergency! Saves taking a lot of what some folks carry in their first-aid kits.