Here are the two states of the right click menu from my browser. The one on the left is when I click on a image and the one on the right when I click on an area that is not an image. The typical right click disable routines disable both of these. I've circled in red those options that I use all the time which I would lose if I allowed the disable routine to be effective:
So when ever someone disables the right click menu, they are in effect attacking me personally by robbing me of options which are available to me in my browser. And as we already know, disabling the right click menu does not really protect the images from being copied, printed, saved, etc. There are other methods of somewhat protecting images that will work better and they should be the first line of defense.