That scenario is so highly unlikely to happen, that it's not worth even thinking about it nor spending money to protect it.
There's people that use the iPhone naked without any protection and their lenses are perfectly in tact. Plus, you'd decrease the quality of your photos/videos by a tiny bit. Not matter how clear the glass on the lens is! Not to mention it looks ugly.
At the very least-- if you work in construction or a higher stress physical job that has rocks everywhere and a lot of dust then MAAAYBE but even then, you're not going to be using your phone over a bunch of rocks.