there are definitely good valid use-cases for wanting to include sitefinity content inside a content block. By doing the
you lose the ability to wrap the content a) inside a container div and b) inside the container content div.
The way I have solved this in the past is by adding support for Shortcodes to be injected into the content html. Here's an introduction: http://www.sitefinity.com/blogs/josh-morales-blog/2012/12/04/supporting-shortcodes-in-sitefinity-content
And here's an example of how you can display a link to a blog post inside a content block, even using the RadToolBar to select one to show: http://www.sitefinity.com/blogs/josh-morales-blog/2012/12/10/creating-shortcodes-for-sitefinity-blog-posts
You could use this technique to embed the blog post summary, or title, or really anything, as well as show an image or document from the library. You simply need to change how the template codebehind parses and replaces the shortcode.
I hope this is helpful!