I can't really see anything that snaps onto a laptop shell is going to do anything other than make it look different. A smaller device like a phone or tablet might be better protected, especially if there's no hinge like a laptop screen.
As far as the whole Apple laptop thing goes, I started with an iBook G3 800 I got around 2002. It used a clear polycarbonate shell on top of white plastic. It was made in Taiwan - I think by Quanta. That thing was rock solid. The performance became less and less acceptable over time, but the case was solid.
After that I got the last generation iBook G4. By then they had gone to white opaque shell and a different kind of plastic around the screen. That did crack on me, but that was well after the warranty was over. My cracks were on the bezel around the screen. My polycarbonate MacBook (2007) has about the same basic appearance, although it's a widescreen. And yeah - that cracked too after maybe 5 years. It still works perfectly, although theoretically Apple says it's obsolete. My wife was using it after her PC became unusable due to assorted malware. The only thing I had to change was the battery after it started bulging. It obviously isn't ideal, but for a 10 year old computer that the manufacturer theoretically doesn't support, it works exceptionally well.
Right now I'm on a MacBook Pro 13" with the aluminum shell. It's the last model with an optical drive. The shell itself isn't likely to crack, but I suppose it could dent. And if it's cracking around the screen, well that's glass so I'd think it would be worse than just a cracked plastic bezel.
