When a Web framework isn't convenient to use?
The difference is, in my opinion, the simpleness of what you are trying to build.
Are you trying to make a difficult, complicated web app, or are you making a simple script that performs a trivial task? In the first case you absolutely need a framework, in the second case.. don't even bother.
When a Web framework ( like django, ruby on rails, zend, etc ) isn't convenient to use ?
And so... When a Web programming language ( like PHP, Asp, Python, etc ) is better than a Web Framework ?