It probably would not help prevent breakage, it would only prevent scratches. It would definitly affect the mirror's performance. I remember having a program on my plam pilot called "Mirror." Basically it made the screen black, so that you could use it as a mirror. Once I applied a screen protector, to much of the reflected light was diffused, and i couldn't use it as a mirror. But those protectors are quite easy to remove (and apply for that matter), so you should be able to keep the protector on it when not in use, and just peel it off when you need to.