In reply to Peter Cralen (@PeterCralen).

Sure it is not general protection and for the most sites its not well solution. Anyway for some situations it does good job. Even these comments from developers confirm that, if they work on site, they ask to turn off these kind of plugins. User will not ask it, he will just search for another source of content rather than fight with developer tool to download for example several pictures, bc. its more easy to find another site, than download every image with firebug or whatever.

In some areas (for example adult content) its really helpful and it helps more than harm (maybe you lost some users with assistive technology, but your content is not used by all your competitors across internet).
You are all right that this is not solution for photographers, there are better way for this.
Generalizing that these kind of plugins are useless is just not right. These functions - plugins have own place and usage for some specific situations.