If kids are typing bad words into a computer you don't fix the problem by taking away the keyboard.

This content cannot be displayed in a frame

To help protect the security of information you enter into this website, the publisher of this content does not allow it to be displayed in a frame.

I use noScript to block pages from doing all kinds of things I do not want, but here is accomplishes the opposite: it prevents me from looking a a page the way I want to.
The usability of sites like google image search will be limited more and more if sites start adding this header. I fear the moment wordpress, joomla, drupal, etc. make this header a default. It will basically make frames useless.

And unfortunately this 'protection' only protects the sites that want to be protected, so it is very limited until it becomes the default way of working.

It should have been a browser setting, not a page header.

Pages that do not let themselves be molded to the wishes of the user, are annoying. That's why we all hate pop-ups, right-click protection, etc. And framebusting.

I know many web publishers find that their webpage should be viewed full screen, but that is not allways what I, the reader, the person they make their site for, want.

I fear that X-FRAME_OPTIONS will become the next weapon for publishers to enforce their wishes upon their readers, so I've come to the conclusion that it is an annoyance, rather than a good thing.