Due to a long, sad history of exploited security defects, I only run javascript from authors I consider both trustworthy and competent, and who have useful behavior to add to my browsers. Basically if you don't even realize your documents are unusable without scripting, I can't assume you're smart enough to avoid attacks via malicious third-party code.

It's also a good way to find out who cares about the web, which is eroding as authors give up publishing semantic markup at stable URLs in favor of content entombed within client/server style apps.