> When you make a buffer read-only (via C-x C-q toggle-read-only), and the file > in the buffer is under version control, you get a warning message: "File is
> under version-control; use C-x v v to check in/out".
> Anyone have any idea why?
I can see one possible reason for this warning message. Some version control systems, Perforce is one example, set all files that are under version control read only by default. If you want to modify a given file, you are supposed to Check Out or Open the File For Edit first. When you use this command, it toggles the read only state of the file off. When you are done with the file you are supposed to submit the change.
Perforce, unlike Subversion and Bazaar, by default does not automatically notice files that you have modified on disk. It only knows about files you have first checked out. So, if you manually toggle the read only state of the file off, Perforce has absolutely no idea that it has changed; the submit command will only submit files you first checked out with the edit command. Thus, by manually toggling the read only state of the file you run the risk of confusing any version control systems that work like this, and, as a result, changes you made may not be submitted as you expect.
This possibility is most likely what lead to the warning message being added.