I have an REI goretex parka that i always have even just walking in the woods. I always carry one of Dougs excellent little Pocket Survival kits, a small First Aid Kit, my SAK, a single AAA cell LED light, my Surefire light and a flask of water. with that minimalist gear I am good for most overnight emergencies.
Normally I have a rucksack with more extensive gear including a poncho and liner, which beats the snot out of those mylar emergency blankets, in my humble opinion.
It only takes a minute for an idyllic outing to turn into a severe survival situation, we cannot plan when that will happen, that is why I chose to be prepared for such things always. But then we all have to have at least one wakeup call in our lives, if we are lucky to survive such things we seldom forget.
In the Pacific NorthWet, with our prodigious and continuous rainfall, hypothermia is a year round killer. It doesn't discriminate and always lies in wait for the weak, unsuspecting, or ignorant. Having the ability and tools to start a fire quickly and efficiently, as well as a means to get out of wind and rain, is mandatory.
It only took me one bad experience where I was borderline hypothermic to always keep that in mind.