When I find a JS-heavy site that people rave about, I like to see how it performs with JS off. Usually it's a disaster, but not always ... so I like to see what makes the difference. It's also a good test before deciding to use some off the shelf script (i.e. how does it perform without JS). I also like to test my own work with JS off if JS has been used to make sure it's usable without JS. And finally, when I'm debugging CSS for people here and their site is using a lot of JS, it can help to turn off JS so that Firebug is usable, as it often jumps all over the place if there is JS happening on the page.
Sometimes, though, I forget to turn JS back on again, and later I might be visiting a site like YouTube and wonder why everything is totally borked.