I believe that's how the Dunwich Dynamo started. Although I'd imagine doing it in jeans would leave your nethers somewhat akin to a freshly tenderised steak.
I've done it in jeans. Admittedly with padded shorts underneath.
One of the guys I met along the way on that occasion was in regular jeans and no padded shorts. He also didn't bring water or food. He made it to the end, with the biggest grin on his face that you can imagine.
Sometimes I like to shoot for comfort, technical wear, the right food, sunglasses for the job, etc.
Other times I just want to remember what it's like to experience that child-like feeling of just grabbing your bike and rushing out for a ride.
There's something really liberating and fun about not acting like an adult and preparing. And it re-introduces some risks that make the ride more 'interesting'... such as whether you'll bonk, and whether you get lost because you didn't plan, or whether it rains and you get soaked to the bone as you didn't check.
Jeans or not, if you ride enough, often enough, hard enough, and long enough, you will
* Bonk.
* Get lost.
* Get soaked/frozen because the weather took a turn for the worse.
* Get roasted/bathed in sweat because it got a lot warmer than you planned on.
I have hours of anecdotes for all of the above, despite dressing in the best possible gear I had/could afford at the time, and riding the best possible bicycle I could afford at the time (gears, brakes, etc...).
http://en.wikipedia.org/wiki/Dunwich_Dynamo