While some of these scripts are effective at disabling the right mouse button and in some cases even keyboard access to the context menu these scripts only achieve that much in some browsers. Other browsers realizing how pointless such a script actually is will ignore any attempt to block the context menu.
What makes using such a script even worse is that it blocks your visitor's access to the other options within the context menu that they may wish to legitimately use to navigate around the web.
Overall using such a script on your web site will drive your legitimate visitors away, not stop any would be thieves from stealing your images or viewing your page source, and will generally show people that you do not understand very much about how the web works. You should never have any reason to use such a script on your site (except where you are providing a working example of how useless such scripts are).