I got one of the glass screen protectors, and it scratched within 2 days. I think it may because women sometimes drop it in our purses without thinking. I have a certain pocket I put it in now in my purse.
Gorilla glass can be scratched by sand and other debris. My last phone had the second generation glass (I think I read the M8 is using generation 3), and had a lot of fine scratches after two years. Nothing too distracting, but scratched none the less. I work in a "dirty" environment where it's easy to pick up dirt and grit. So GC scratching is one of those YMMV things. Even if I stayed indoors all day, I'd still use a protector now that I know it's not as impervious as they would have you believe. It's durable, but not perfect. As others have said, it's better to replace a protector if needed, than to replace the actual screen.