AFAIK all modern browsers offer the ability to turn off JavaScript globally. Add-ons such as NoScript for Firefox give you more control though. I use NoScript to have all JS disabled by default on a page, and turn on/enable specific sources as necessary. On zencastr, I noticed that the page wasn't loading so I temporarily white-listed the site, and it is visible now.

A common and, IMO not-unreasonable view is that all content on the web should be viewable on the web with just HTML and CSS (sometimes even without the latter) and thus JS should never be required save for specific tasks such as in-browser computation and the like. That's a bit extreme for me, so I just white-list scripts as needed.

I use NoScript for security reasons. I find that I usually only have to turn on 1 or 2 scripts to get the content on the site to show. If even that as many show it without scripts. It's a tiny percentage, often using fancy crap underneath, that absolutely require JavaScript to even see static content. I don't loose much ignoring them but gain plenty in security, loading speed, and battery savings.

You disable it in your browser or most likely run something like noscript to maintain a white list.

Around 1% of users run without js. If you have a commercial site you will loose money and users if you don't think about this.