In all seriousness, though. ....
Of course there is always some sort of wild emergency that could arise... Russians invading, tornado touchdown, etc....
Falling and breaking an ankle. Cutting yourself badly and needing to go for stitches. Getting dehydrated, falling, and hitting your head.
In all seriousness, those are the in-betweens that people carry ID and insurance info for.
While this is generally true throughout the States, Disney has it's own policy that differs. It started last year and required a passport as ID for non Americans when purchasing alcohol. The reason was that CMs couldn't always understand the language some DLs were issued in. Not all foreign guests have DLs written in English. But it wasn't universally enforced. Recently I've read reports where they were content with a photocopy of the first page (photo page) of the passport. This was in response to the (IMO reasonable) objection of guests who were very reluctant to carry their passport around with them in the parks and risking losing them.
That's the sequence of events I've read! For me, if I ever go to Tokyo Disney I'll carry my passport with me. So I never really get the reluctance to carry THE international ID, but I do get that some people are reluctant.
Far enough to get back to my hotel room I suppose, which is where my ID would be safely stowed in my safe.
That sounds like a fun couple of hours (and since my family has spectacularly bad luck with transportation at disney, so the reality for us is that would mean the end of the park day after fighting to get back to the room).
I also had an ulterior reason for the question with regard to my excellent 21 year old stepson, whose one shortfall might be misplacing things. I might like to be able to tell him not to bring his wallet to the parks as it could get...
1. Lost
2. Soaked
http://www.rei.com/product/723131/witz-see-it-safe-sport-case
This carries my ID, insurance card, CC (b/c we generally don't charge to the rooms but even when we do I've had the system fail so I keep a CC with me), DVC card, AP card, and a bit of cash (though it gets crowded when I get change!). Basically waterproof and very convenient. It's what I used pre-MB and it's what I still use.
I'm 44, I still get carded often (and not always in a pitying way, LOL), and I know that if you are asked for ID and don't have it, you're not getting that beer (even if you're 98) because they could get into trouble if they served you.
Answers that go beyond "yes or no" are conversation that happens. There are many other people who read threads beyond the original poster, and the info can be good for them, even if it's annoying to others. Plenty of us have had things happen or know of things that can happen, that are far less than catastrophic, that would make us want to have our IDs.