Real quick, the lens you can see and touch on an iPhone is a lens cover. Its purpose is to protect the actual camera lens behind it. Apple uses sapphire crystal because its highly transparent and highly scratch resistant.
Protecting the lens cover (protecting the protector?) can be more harmful than helpful.
Keep in mind dust produced by grinding, cutting, sanding and drilling (abrasives and cutting materials) contain aluminum oxide, tungsten carbide, etc. Even driving down a gravel road will kick up aluminum oxide, and then slamming on your brakes to stop will cause brake dust which is filled with silicon carbide (mostly high performance brakes). Those are just a few easy sources of dust that contains minerals as hard and/or harder than Apples sapphire lens which means they can be abrasive too it.
Typically this isn't a problem. The problem is when you make a place for dust to rub between things like between a lens and a lens protector.
Degraded imagine quality would be the main problem though. Most of those protectors are tempered glass (or even plastic) which can scratch much easier than sapphire. Adding another lens also adds two additional surfaces for dust and micro scratches which will make the image softer and reduce transparency. The lens protector will catch some of the light cause images to look a bit washed out. Typically its not until it gets really bad that a lens protector is removed revealing much better images leaving the owner with weeks, months, even years of sub optimal photos.
If you are worried about it get a lens protector with camera cutouts OR a case that recesses the camera bump.
Good luck!
