I'm +1 on adding such an option to the menu, if only to be able to highlight files including only Python code which don't happen to have the .py extension.

If support for files including not only Python code is important to anyone, some examples of such files would be very helpful. I've never seen such files and I don't know how common such uses are.

Supporting files with some Python code in between other stuff is a different matter. Non-Python code being wrongly highlighted may be fine, but we'll need to make sure that the syntax highlighting is robust enough to be able to withstand going through a lot of invalid syntax, and still correctly highlight most of the Python code found in between. If IDLE fails to highlight a file it could of course fail gracefully, give an error message and not highlight, so I don't foresee this causing any stability issues.