The GoPro Hero 3 lens protrudes from its housing, making it vulnerable to damage (i.e. scratching). We purchased this in-expensive Lens Cover to protect the lens and have found it to be very effective.
Only problem with the GoPro Hero lens cover its that it blends into the unit too well, making it easy to leave on the unit when filming. I have done this several times! - user error I know but frustrating. Would be great if it was a bright colour for oldies like us! I might have to get the paint out.