Responding to this specific part of the question: "scratches caused by fingernails or creating worn out areas of the screen, due to frequent use." Although many things can and will degrade the coating (oleophobic or other), the gorilla glass itself will not be scratched by fingernails. It won't even be scratched by car keys in the same pocket. In normal use, the most common thing that scratches mobile phone glass is sand or some types of grit. Whether on the beach, in the car or in your pocket, your phone will meet some of this stuff - it's everywhere. If you know you've got some, make sure the phone is dry and blow it out of any crevice with air. Under no circumstances wipe the glass if there are any particles of sand or grit on it.
As for protectors, one problem is that they use a small amount of adhesive which, either on application or in use, can attract particles which will degrade the glass. The real question is whether you are prepared to accept what will often be a major deterioration in tactile response when you swype or slide your finger on the surface compared to the minimal protection offered by many protectors. Some users don't find the protector reduces the performance. Some find the protector so diminishes use of a light touch that they simply won't consider them. I can't quote accurate statistics, but in my own experience, users who can't use protectors at all tend to be ones that either use a very light touch or have to press hard on the glass in the first place to get a response. Others I know that refuse to use protectors tend to be swipers rather than tappers. Not a scientific survey and highly subjective, but it gives you an idea that you may not get on with a protector.