Note that "Disabled, enable when needed" is meant to include having two different browsers, one with JS, one without and picking between the two depending on what you are doing.
I usually browse with JS enabled. But when JS slows down a site, or is used for annoying things like text that follows the mouse, scrolling titles or statusbar texts, I disable it by unchecking a checkbox. I have Java disabled, but when needed it can be enabled with a similar checkbox. Etcetera... :)