I think screen protectors fall into the category of products that exist so you can pay mostly for your own peace of mind, without much actual benefit.
99% of the key marks you see on the screen are only finger oils and grime, not actual scratches. They can be removed using the method Apple recommends: A clean, moist cloth. I've owned four Apple laptops and never bought a screen protector.
The only time it might be a good idea is if you often carry your laptop so that it's always squeezed under pressure between books in a bag, but frankly, if you're carrying your laptop that way...you shouldn't be, it'll cause bigger and more expensive problems than just a scratched screen!