And it would make users like me leave the site without ever looking back. My browser uses
Mouse-Gestures to navigate. Use right-click->open in new tab a lot. Disabling right mouse button has nothing to do with copying content (don't need a mouse for that) but all with normal navigation.
PS The only people you might stop are real web-novices, and i think it is unlikely these are the people that would copy&paste things to other sites to start with.
My conclusion: It won't stop anyone from copying, but it will drive away more experienced users from your site.