I am endorsing this ripstop nylon camo pattern version of the poncho, but what I write is applicable in general to just about all heavier-duty nylon, rubber or urethane coated, military or outdoors camping-grade ponchos.  Almost every military in the world issues them.  There are many high-quality commercial brands (including the ones sold here).  But whatever the maker/origin, they are one of the most practical and useful items to have in your kit, IMHO.

I was just thinking about this as the rainy season just started here and I attached my "rain/cold weather" MOLLE pouch to my belt.  My belt pouch module (6" X 6" X 3") contains my rain poncho, an emergency bivy sack, a Nomex hood (which rolls up like a watch cap), and a chemical hand-warmer packet.  These items added to what I already carry everyday, make me prepared for inclement weather or even an unscheduled night out in the elements.

A poncho is not much more than a large piece of water-proof cloth (or vinyl) with a headhole that allows it to be worn, completely covering the front and back of the body (including equipment and pack).  The technology dates back to the American Civil War when Union soldiers were issued "Gum Blankets" which were pieces of canvas coated with a rubber mixture (made by "Goodyear" believe it or not!).  It is very effective as raingear.  Ah!  but a poncho can be so much more.

Though not the most effective at this function, a poncho is a rudimentary (minimally insulative) blanket.  When used in combination with a quilted poncho liner, it becomes a very effective blanket.

When laid out on the ground, it's a nice waterproof ground cloth.  Basically like any tarpaulin.

When snapped into a tube, it forms a simple sleeping bag.

This same tube makes a field expedient body bag.

In Ranger School, they teach a technique by which two ponchos can be sealed together to make a pontoon float that can buoy two full rucksacks.

A poncho is very effective for smoke signalling (trapping and accumulating smoke).

When suspended or supported overhead, it makes a good shelter (or tent).  It can also protect (shelter) individual objects sensitive to moisture.

This same shelter idea can be used to collect water (rain) in a survival situation, channeling into a basin or canteen.

Similarly, it can be hung up as a wind/sun shelter or blind to conceal something.

It's wind catching ability can be capitalized on to form a sail, if lost at sea or on open water.

The technique is not easily explained (in text), but a poncho (and many sticks) can be used to make an effective "fish trap" in a survival situation.

Like any large piece of cloth, it can be an emergency catch blanket for someone to jump down into from a height.

Like any large piece of cloth, it can be a drag sled or litter (for dragging game, or tranporting an injured person).

Similarly, it can be used to carry large, heavy loads of anything (firewood, dirt, stones, ammo, forage).

If rolled, folded correctly, it can carry several gallons of water in one load for short distances.

Although I wouldn't trust my weight on it as a "hammock", I have personally used a poncho along with rope and paracord to help hold/haul loads up into a tree blind and to suspend food away from bears and critters.

With tent poles or any flexible "poles" of sufficient length, one can make an Alpha Tent (low dome personal bivy)out of a poncho by putting the end of the tent poles through the opposite (not adjacent) corner grommets of the poncho.

Looking at Native American "shirts" in museums, one can see that the poncho idea (a headhole through an animal skin) is the basis for much early clothing.  One of the great things about a poncho as clothing is that it can be easily improvised (like in Rambo: First Blood).  A rectangular piece of cloth, headhole and some rope to belt it off.  And anything waterproof, like a trash can liner, upholstery, animal skin or tarp, can become poncho for raingear.

Similar to the clothing concept, the poncho forms a basic cape, and if the poncho is camofluaged or matches the environmental colours, it makes a decent "mobile" sniper veil.  This variant type of poncho is actually an issue item in Eastern Bloc armies.

I know this is silly, but once, I had occasion to use my grey poncho as a field expedient slide projector screen out in the jungle.

I'm sure there are many more functions for a poncho, but I'll leave it at that for now.  Bottomline is, with all the very useful, potentially life-saving uses of this simple device, I think it's an excellent investment for anyone's "Emergency Preparedness Kit".  So why not add one?