They use them: a) to improve page load times by serving resources the user's browser may have already cached b) to serve ads c) to track usage so that they c

They use them: a) to improve page load times by serving resources the user's browser may have already cached b) to serve ads c) to track usage so that they can optimize advertising and conversions d) in some cases, to implement A/B testing experiments without having to hard-code them into the site The downsides you mention are pretty minimal. For the vast majority of sites: a) NoScript users are a teensy minority so small as to be statistically insignificant and b) the site is FAR more likely to have downtime of its own infrastructure than for Google or Facebook or whatever other CDN to go down.