Ok, so I don't put a case around my windscreen. Happy now?Terrible examples. Your seats or couch you clean when get dirty. Plastic sheeting on them offers no protection; if you step wrong on the fabric it can still rip easily at the seams etc.
You cannot fix broken glass without replacing it, once its gone its gone. The case/screen protector isnt protecting a phone from getting dirty but trying to minimize shattering said front and back glass.
And even with a case and screen protector you still get breakage....You may avoid the 'dirt' from the micro scratches although a case is debatable as it suggests there is often enough actually more risk to damage when using one...